misc/pvpanic: add support to get pvpanic device info FDT

By default, when ACPI tables and FDT coexist for ARM64,
current kernel takes precedence over FDT to get device information.
Virt machine in qemu provides both FDT and ACPI table. Increases the
way to get information through FDT.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>