arm64: kdump: Support crashkernel=X fall back to reserve region above DMA zones

For crashkernel=X without '@offset', select a region within DMA zones
first, and fall back to reserve region above DMA zones. This allows
users to use the same configuration on multiple platforms.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20221116121044.1690-3-thunder.leizhen@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>