arm64/kexec: Use consistent convention of initializing 'kxec_buf.mem' with KEXEC_BUF_MEM_UNKNOWN

With commit b6664ba4 ("s390, kexec_file: drop arch_kexec_mem_walk()"),
we introduced the KEXEC_BUF_MEM_UNKNOWN macro. If kexec_buf.mem is set
to this value, kexec_locate_mem_hole() will try to allocate free memory.

While other arch(s) like s390 and x86_64 already use this macro to
initialize kexec_buf.mem with, arm64 uses an equivalent value of 0.
Replace it with KEXEC_BUF_MEM_UNKNOWN, to keep the convention of
initializing 'kxec_buf.mem' consistent across various archs.

Cc: takahiro.akashi@linaro.org
Cc: james.morse@arm.com
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>