arm64: introduce VA_START macro - the first kernel virtual address.

In order to not use lengthy (UL(0xffffffffffffffff) << VA_BITS) everywhere,
replace it with VA_START.

Signed-off-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-Commit: 127db024a7baee9874014dac33628253f438b4da
Git repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>