Add RGMII to supported interfaces for BCM63xx RGMII ports so they can be
actually used in RGMII mode.
Without this, phylink will fail to configure them:
[ 3.580000] b53-switch 10700000.switch GbE3 (uninitialized): validation of rgmii with support 0000000,00000000,00000000,000062ff and advertisement 0000000,00000000,00000000,000062ff failed: -EINVAL
[ 3.600000] b53-switch 10700000.switch GbE3 (uninitialized): failed to connect to PHY: -EINVAL
[ 3.610000] b53-switch 10700000.switch GbE3 (uninitialized): error -22 setting up PHY for tree 0, switch 0, port 4
Fixes: ce3bf94871f7 ("net: dsa: b53: add support for BCM63xx RGMIIs")
Signed-off-by: Jonas Gorski <jonas.gorski@xxxxxxxxx>