Expand POST memory test to support arch-depended implementation.

Add weak functions to enable architecture depended preparation, address
advancing, cleaning up and error handling.

These weak functions provides the framwork to implemente arch/platform
dependent code for initializing/maintenance/restore the start address, size,
physical address as well as memory mapping before/between/after memory test.
arch_memory_failure_handle can also be implemented in case more care is needed
for arch/platform.

Signed-off-by: York Sun <yorksun@freescale.com>