arm64: kdump: Provide default size when crashkernel=Y,low is not specified

Try to allocate at least 128 MiB low memory automatically for the case
that crashkernel=,high is explicitly specified, while crashkenrel=,low
is omitted. This allows users to focus more on the high memory
requirements of their business rather than the low memory requirements
of the crash kernel booting.

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/20221116121044.1690-2-thunder.leizhen@huawei.com


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