arm64: Add support for 48-bit VA space with 64KB page configuration

This patch allows support for 3 levels of page tables with 64KB page
configuration allowing 48-bit VA space. The pgd is no longer a full
PAGE_SIZE (PTRS_PER_PGD is 64) and (swapper|idmap)_pg_dir are not fully
populated (pgd_alloc falls back to kzalloc).

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Jungseok Lee <jungseoklee85@gmail.com>

diff --cc Documentation/arm64/memory.txt
Simple merge

diff --cc arch/arm64/Kconfig
Simple merge

diff --cc arch/arm64/include/asm/page.h
Simple merge

diff --cc arch/arm64/kernel/head.S
Simple merge