powerpc: Enable CONFIG_KEXEC_FILE in powerpc server defconfigs.

Enable CONFIG_KEXEC_FILE in powernv_defconfig, ppc64_defconfig and
pseries_defconfig.

It depends on CONFIG_CRYPTO_SHA256=y, so add that as well.

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