arch: arm: atmel_sam: Fixup build issue on sam_e70

With recent dts script change we dropped _0 from define names if there
is only a single one.  So for the MPU we need to use
CONFIG_SRAM_BASE_ADDRESS instead of CONFIG_SRAM_BASE_ADDRESS_0.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>