powerpc: Fix 32-bit SMP boot on CHRP

prom_init was changed to take a new argument, the address
where the kernel is loaded, which is now used to copy the
SMP spin loop down before use.

However, only head_64.S was adapted to pass this new value,
not head_32.S, thus breaking SMP boot on 32-bit SMP CHRP
machines.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>