ARM: at91: add sama5d2 smc header

Add a header for SAMA5D2 SMC since it's not compatible with
SAMA5D3 one.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
[wenyou: fix the wrong base address of the SMC register]
Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>