arm64: remove dead code

Commit 68234df4 ("arm64: kill flush_cache_all()") removed
soft_reset() from the kernel. This was the only caller of
setup_mm_for_reboot(), so remove that also.

Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>