UPSTREAM: arm64: choose memstart_addr based on minimum sparsemem section alignment

This redefines ARM64_MEMSTART_ALIGN in terms of the minimal alignment
required by sparsemem vmemmap. This comes down to using 1 GB for all
translation granules if CONFIG_SPARSEMEM_VMEMMAP is enabled.

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

Change-Id: I05b8bc6ab24f677f263b09d7c31fcce4f21269b1
(cherry picked from commit 06e9bf2f)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>