at91sam9rlek_mmc_defconfig: Add CONFIG_ARCH_AT91=y

This flag was missing and thus the board was totally being configured
wrong.

Signed-off-by: Tom Rini <trini@konsulko.com>

configs/at91sam9rlek_mmc_defconfig

 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9RL,SYS_USE_MMC"
 CONFIG_ARM=y
+CONFIG_ARCH_AT91=y
 CONFIG_TARGET_AT91SAM9RLEK=y
 CONFIG_HUSH_PARSER=y