net: ethernet: cadence-macb: Add disabled usrio caps

On some platforms, the macb integration does not use the USRIO
register to configure the (R)MII port and clocks.
When the register is not implemented and the MACB error signal
is connected to the bus error, reading or writing to the USRIO
register can trigger some Imprecise External Aborts on ARM platforms.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>