net: macb: remove #if defined(CONFIG_ARCH_AT91) sections

With multi platform support those sections could lead to unexpected
behavior if both ARCH_AT91 and another ARM SoC using the MACB IP are
selected.
Add two new capabilities to encode the default MII mode and the presence
of a CLKEN bit in USRIO register.
Then define the appropriate config for IPs embedded in at91 SoCs.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>