arm64/kexec: Add pr_debug output

To aid in debugging kexec problems or when adding new functionality to
kexec add a new routine kexec_image_info() and several inline pr_debug
statements.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>