net: phy: broadcom: add helper to write/read RDB registers

RDB (Register Data Base) registers are used on newer Broadcom PHYs. Add
helper to read, write and modify these registers.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>