armv8: allow custom MMU setup routines on ARMv8

armv8: allow custom MMU setup routines on ARMv8

In order for noncached_init() to operate correctly, SoCs must set up a
custom page table with fine-grained (2MiB) sections, which can be
configured from noncached_init().

This is currently performed by arch/arm/cpu/armv8/{fsl-lsch3,zynqmp}/cpu.c
by cut/pasting and re-implementing mmu_setup, enable_caches(), etc. There
are some other reasons for the duplication there though, such as enabling
icache early, and enabling dcaching earlier with a different configuration.

This change makes mmu_setup() a weak implementation, so that the MMU setup
code can be replaced without having to duplicate other code that calls it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>