ARM: DRA7xx: Fix memory allocation overflow

When using early malloc the allocated memory can overflow into the SRAM
scratch space, move NON_SECURE_SRAM_IMG_END down a bit to allow more
dynamic allocation at the expense of a slightly smaller maximum image
size.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>