dt-bindings: net: dwmac: Document STM32 property st,ext-phyclk

dt-bindings: net: dwmac: Document STM32 property st,ext-phyclk

The Linux kernel dwmac-stm32 driver currently supports three DT
properties used to configure whether PHY clock are generated by
the MAC or supplied to the MAC from the PHY.

Originally there were two properties, st,eth-clk-sel and
st,eth-ref-clk-sel, each used to configure MAC clocking in
different bus mode and for different MAC clock frequency.
Since it is possible to determine the MAC 'eth-ck' clock
frequency from the clock subsystem and PHY bus mode from
the 'phy-mode' property, two disparate DT properties are
no longer required to configure MAC clocking.

Linux kernel commit 1bb694e20839 ("net: ethernet: stmmac: simplify phy modes management for stm32")
introduced a third, unified, property st,ext-phyclk. This property
covers both use cases of st,eth-clk-sel and st,eth-ref-clk-sel DT
properties, as well as a new use case for 25 MHz clock generated
by the MAC.

The third property st,ext-phyclk is so far undocumented,
document it.

Below table summarizes the clock requirement and clock sources for
supported PHY interface modes.
 __________________________________________________________________________
|PHY_MODE | Normal | PHY wo crystal|   PHY wo crystal   |No 125Mhz from PHY|
|         |        |      25MHz    |        50MHz       |                  |

---------------------------------------------------------------------------
|  MII    |    -   |     eth-ck    |        n/a         |       n/a        |
|         |        | st,ext-phyclk |                    |                  |

---------------------------------------------------------------------------
|  GMII   |    -   |     eth-ck    |        n/a         |       n/a        |
|         |        | st,ext-phyclk |                    |                  |

---------------------------------------------------------------------------
| RGMII   |    -   |     eth-ck    |        n/a         |      eth-ck      |
|         |        | st,ext-phyclk |                    | st,eth-clk-sel or|
|         |        |               |                    | st,ext-phyclk    |

---------------------------------------------------------------------------
| RMII    |    -   |     eth-ck    |      eth-ck        |       n/a        |
|         |        | st,ext-phyclk | st,eth-ref-clk-sel |                  |
|         |        |               | or st,ext-phyclk   |                  |

---------------------------------------------------------------------------

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Christophe Roullier <christophe.roullier@foss.st.com>
Link: https://lore.kernel.org/r/20240328185337.332703-2-christophe.roullier@foss.st.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>