net: phy: broadcom: move shadow 0x1C register accessors to brcmphy.h

The shadow register 0x1C is used both by the BCM54xxx PHYs and the
BCM7xxx internal PHYs, move the accessors to a common location so both
drivers can use them.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>