dt-bindings: mfd: Convert lpc1850-creg-clk, pc1850-dmamux and phy-lpc18xx-usb-otg...

dt-bindings: mfd: Convert lpc1850-creg-clk, pc1850-dmamux and phy-lpc18xx-usb-otg to YAML format

Combine the following separate plain text based bindings to YAML:

  lpc1850-creg-clk.txt
  pc1850-dmamux.txt
  phy-lpc18xx-usb-otg.txt

Additional changes:

- remove label in example.
- remove dmamux consumer in example.
- remove clock consumer in example.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: "Rob Herring (Arm)" <robh@kernel.org>
Link: https://lore.kernel.org/r/20250602143612.943516-1-Frank.Li@nxp.com
Signed-off-by: Lee Jones <lee@kernel.org>