rtc-cmos.c : Fix warning on PowerPC

The following warning is seen while compilation of PowerPC kernel:

 CC      drivers/rtc/rtc-cmos.o
drivers/rtc/rtc-cmos.c:697:2: warning: #warning Assuming 128 bytes
of RTC+NVRAM address space, not 64 bytes.

Fix it by adding defined(__powerpc__).

Signed-off-by: Srikanth Krishnakar <skrishna@mvista.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>