Revert "arm64: account for sparsemem section alignment when choosing vmemmap offset"

This reverts commit 36e5cd6b

, since the
section alignment is now guaranteed by construction when choosing the
value of memstart_addr.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>