net: dsa: rtl8365mb: add GMII as user port mode

Recent net-next fails to initialize ports with:

 realtek-smi switch: phy mode gmii is unsupported on port 0
 realtek-smi switch lan5 (uninitialized): validation of gmii with
 support 0000000,00000000,000062ef and advertisement
 0000000,00000000,000062ef failed: -22
 realtek-smi switch lan5 (uninitialized): failed to connect to PHY:
 -EINVAL
 realtek-smi switch lan5 (uninitialized): error -22 setting up PHY
 for tree 1, switch 0, port 0

Current net branch(3dd7d40b) is not
affected.

I also noticed the same issue before with older versions but using
a MDIO interface driver, not realtek-smi.

Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>