powerpc/ps3: Add missing check for PS3 to rtc-ps3 platform device registration

On non-PS3, we get:

| kernel BUG at drivers/rtc/rtc-ps3.c:36!

because the rtc-ps3 platform device is registered unconditionally in a kernel
with builtin support for PS3.

Reported-by: Sachin Sant <sachinp@in.ibm.com>
Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>