net: phy: marvell: Add functions to read PHY's extended registers

This commit allows extended Marvell registers to be read with:

foo > mdio rx FEC 3.10
Reading from bus FEC
PHY at address 0:
3.16 - 0x1063
foo > mdio wx FEC 3.10 0x1011

The above code changes the way ETH connector LEDs blink.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: York Sun <york.sun@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>