xtensa: Switch to sched_clock_register()

The 32 bit sched_clock interface now supports 64 bits. Upgrade
to the 64 bit function to allow us to remove the 32 bit
registration interface.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>