[PowerPC] Fix setting of time in OPAL

There were multiple bugs in the OPAL RTC code which had never been
discovered, as the default configuration of OPAL machines is to
have the BMC / FSP control the RTC.

• Fix calling convention for setting the time -- the variables are passed

directly in CPU registers, not via memory.

• Fix bug in the bcd encoding routines. (from jhibbits)

Tested on POWER9 Talos II (BE) and POWER9 Blackbird (LE).

Reviewed by: jhibbits (in irc)
Sponsored by: Tag1 Consulting, Inc.