arm64: kexec: make dtb_mem always enabled

Currently, dtb_mem is enabled only when CONFIG_KEXEC_FILE is
enabled. This adds ugly ifdefs to c files.

Always enabled dtb_mem, when it is not used, it is NULL.
Change the dtb_mem to phys_addr_t, as it is a physical address.

Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Will Deacon <will@kernel.org>