armv8: enable compilation with CONFIG_SYS_NONCACHED_MEMORY

The implementation of noncached_init() uses define MMU_SECTION_SIZE.
Define this on ARM64.

Move the prototype of noncached_{init,alloc}() to a location that
doesn't depend on !defined(CONFIG_ARM64).

Note that noncached_init() calls mmu_set_region_dcache_behaviour() which
relies on something having set up translation tables with 2MB block size.
The core ARMv8 MMU setup code does not do this by default, but currently
relies on SoC specific MMU setup code. Be aware of this before enabling
this feature on your platform!

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