[POWERPC] Fix drivers/rtc/Kconfig for powerpc

The powerpc arch uses CONFIG_PPC instead of CONFIG_POWERPC.  This fixes
drivers/rtc/Kconfig to use the correct config option.

Signed-off-by: Wade Farnsworth <wfarnsworth@mvista.com>
Acked-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>