RDB (Register Data Base) registers are used on newer Broadcom PHYs. Add
helper to read, write and modify these registers.
Signed-off-by: NMichael Walle <michael@walle.cc>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>