ARN: at91: introduce SOC_AT91xxx define to allow to compile SoC core support

We can now compile all SoC core support together and DT boards.
We still can not compile together the non DT board.
So We keep the ARCH_AT91xxx for the non DT board and for backward defconfig
compatibility. This will enable the plaform_device ressources.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>