net: phy: broadcom: add support for BCM54210E

It's Broadcom PHY simply described as single-port
RGMII 10/100/1000BASE-T PHY. It requires disabling delay skew and GTXCLK
bits.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>