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>

Documentation/arm64/memory.txt

@@ -45,6 +45,14 @@ Start			End			Size		Use
 fffffc0000000000	ffffffffffffffff	   4TB		kernel
 
 
+AArch64 Linux memory layout with 64KB pages + 3 levels:
+
+Start			End			Size		Use
+-----------------------------------------------------------------------
+0000000000000000	0000ffffffffffff	 256TB		user
+ffff000000000000	ffffffffffffffff	 256TB		kernel
+
+
 For details of the virtual kernel memory layout please see the kernel
 booting log.