net: mdio-mux-mmioreg: Add support for 16bit and 32bit register sizes

In order to support PHY switching on Amlogic GXL SoCs, add support for
16bit and 32bit registers sizes.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>