xtensa: remove platform rtc hooks

platform_get/set_rtc_time() is not implemented by any of the supported
xtensa platforms.  Remove the facility completely.

The initial seconds for xtime come from read_persistent_clock() which
returns just 0 in the generic implementation.  Platforms that sport a
persistent clock can implement this function.

This is needed to implement the ccount as a clock source.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Chris Zankel <chris@zankel.net>