pseries: Move sPAPR RTC code into its own file

pseries: Move sPAPR RTC code into its own file

At the moment the RTAS (firmware/hypervisor) time of day functions are
implemented in spapr_rtas.c along with a bunch of other things.  Since
we're going to be expanding these a bit, move the RTAS RTC related code
out into new file spapr_rtc.c.  Also add its own initialization function,
spapr_rtc_init() called from the main machine init routine.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>