[PATCH] powerpc: Make sure we have an RTC before trying to adjust it

Its valid for ppc_md.set_rtc_time to be NULL.  We need to check
that its non-NULL before trying to update the RTC.

Signed-off-by: Kumar K. Gala <kumar.gala@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>