OpenRISC: Timekeeping

Implements support for the OpenRISC timer which is a 28 bit cycle counter
that can be read out of a special purpose register.  This counter is
used as a both a clock event and clocksource device.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Cc: tglx@linutronix.de
Reviewed-by: Arnd Bergmann <arnd@arndb.de>