powerpc: Set CONFIG_32BIT on ppc32

commit ffe8018c of the -mm tree
fixes the initramfs size calculation for e.g. s390 but breaks it
for 32bit architectures which do not define CONFIG_32BIT.

This patch fix the problem for PPC32 which will elsewise end up
with a __initramfs_size of 0.

Signed-off-by: Kerstin Jonsson <kerstin.jonsson@ericsson.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>