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>

arch/arm/cpu/armv8/cache_v8.c

@@ -32,7 +32,7 @@ inline void set_pgtable_table(u64 *page_table, u64 index, u64 *table_addr)
 }
 
 /* to activate the MMU we need to set up virtual memory */
-static void mmu_setup(void)
+__weak void mmu_setup(void)
 {
 	bd_t *bd = gd->bd;
 	u64 *page_table = (u64 *)gd->arch.tlb_addr, i, j;