package/kexec: enable powerpc64 platform

kexec has fully support of ppc64 platform:
https://www.kernel.org/doc/Documentation/kdump/kdump.txt


Added BR2_powerpc64 platform support.

Signed-off-by: Artem Senichev <artemsen@gmail.com>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>