powerpc/kexec: Enable kexec_file_load() syscall

Define the Kconfig symbol so that the kexec_file_load() code can be
built, and wire up the syscall so that it can be called.

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