net: macb: fix default configuration for GMAC on AT91

On AT91 SoCs, the User Register (USRIO) exposes a switch to configure the
"Reduced" or "Traditional" version of the Media Independent Interface
(RMII vs. MII or RGMII vs. GMII).
As on the older EMAC version, on GMAC, this switch is set by default to the
non-reduced type of interface, so use the existing capability and extend it to
GMII as well. We then keep the current logic in the macb_init() function.

The capabilities of sama5d2, sama5d4 and sama5d3 GEM interface are updated in
the macb_config structure to be able to properly enable them with a traditional
interface (GMII or MII).

Reported-by: Romain HENRIET <romain.henriet@l-acoustics.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>