arm: asm/cache.c: Introduce arm_reserve_mmu

As a preparation for turning reserve_mmu into an arch-specific variant,
introduce arm_reserve_mmu on ARM. It implements the default routine for
reserving memory for MMU TLB and needs to be weakly defined in order to allow
for machines to override it.

Without this decoupling, after introducing arch_reserve_mmu, there would be two
weak definitions for it, one in common/board_f.c and one in
arch/arm/lib/cache.c.

Signed-off-by: Ovidiu Panait <ovpanait@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>