arm64: mm: Remove bit-masking optimisations for PAGE_OFFSET and VMEMMAP_START

Currently there are assumptions about the alignment of VMEMMAP_START
and PAGE_OFFSET that won't be valid after this series is applied.

These assumptions are in the form of bitwise operators being used
instead of addition and subtraction when calculating addresses.

This patch replaces these bitwise operators with addition/subtraction.

Signed-off-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>