net: stmmac: Refactor VLAN implementation

net: stmmac: Refactor VLAN implementation

Refactor VLAN implementation by moving common code for DWMAC4 and
DWXGMAC IPs into a separate VLAN module. VLAN implementation for
DWMAC4 and DWXGMAC differs only for CSR base address, the descriptor
for the VLAN ID and VLAN VALID bit field.

The descriptor format for VLAN is not moved to the common code due
to hardware-specific differences between DWMAC4 and DWXGMAC.

For the DWMAC4 IP, the Receive Normal Descriptor 0 (RDES0) is
formatted as follows:
    31                                                0
      ------------------------ -----------------------
RDES0| Inner VLAN TAG [31:16] | Outer VLAN TAG [15:0] |
      ------------------------ -----------------------

For the DWXGMAC IP, the RDES0 format varies based on the
Tunneled Frame bit (TNP):

a) For Non-Tunneled Frame (TNP=0)

    31                                                0
      ------------------------ -----------------------
RDES0| Inner VLAN TAG [31:16] | Outer VLAN TAG [15:0] |
      ------------------------ -----------------------

b) For Tunneled Frame (TNP=1)

     31                   8 7                3 2      0
      --------------------- ------------------ -------
RDES0| VNID/VSID           | Reserved         | OL2L3 |
      --------------------- ------------------ ------

The logic for handling tunneled frames is not yet implemented
in the dwxgmac2_wrback_get_rx_vlan_tci() function. Therefore,
it is prudent to maintain separate functions within their
respective descriptor driver files
(dwxgmac2_descs.c and dwmac4_descs.c).

Signed-off-by: Boon Khai Ng <boon.khai.ng@altera.com>
Link: https://patch.msgid.link/20250507063812.34000-2-boon.khai.ng@altera.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>