powernv: Remove get/set_rtc_time when they are not present

Currently we continue to poll get/set_rtc_time even when we know they
are not working.

This changes it so that if it fails at boot time we remove the ppc_md
get/set_rtc_time hooks so that we don't end up polling known broken
calls.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>