clocksource/drivers/dbx500: Demote dbx500 PRCMU clocksource

clocksource/drivers/dbx500: Demote dbx500 PRCMU clocksource

Demote the DBx500 PRCMU clocksource to quality 100 and
mark it as NONSTOP so it will still be used for
timekeeping across suspend/resume.

The Nomadik MTU timer which has higher precision will
be used when the system is up and running, thanks to
the recent changes properly utilizing the suspend
clocksources.

This was discussed back in 2011 when the driver was
written, but the infrastructure was not available
upstream to use this timer properly. Now the
infrastructure is there, so let's finalize the work.

Cc: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>