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>
(cherry picked from commit 127db024a7baee9874014dac33628253f438b4da)
Signed-off-by: Alex Shi <alex.shi@linaro.org>