powerpc: Add purgatory for kexec_file_load() implementation.

This purgatory implementation is based on the versions from kexec-tools
and kexec-lite, with additional changes.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>