arm64: memory: Simplify _VA_START and _PAGE_OFFSET definitions

Rather than subtracting from -1 and then adding 1, we can simply
subtract from 0.

Tested-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>