net: phy: b53: Add b53_reg read/write commands

Add a b53_reg read/write command which allows inspecting the switch
registers. Because the Broadcom BCM53xx registers have different sizes,
we need to split the accesses in 8, 16, 32, 48 or 64 bits to obtain
expected results.

Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>