arm: aarch32: cortex_a_r: Add MMU_ALIGN define to Linker script

cortex_a_r lacks of MMU_ALIGN definition. This define is added to the
target linker script when CONFIG_NOCACHE_MEMORY is enabled which
adds .nocache section where this define is used.

Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com>