ARM: at91: Add the SoC options to Kconfig

To prepare to remove the SoCs options such as SAMA5D2, SAMA5D3
and SAMA5D4 from the CONFIG_SYS_EXTRA_OPTIONS option which is
deprecated, add the SoC options to Kconfig.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>