sh: First step at generic timeofday support.

At the moment we wrap GENERIC_TIME around our existing timer API.
As boards start providing their own clocksources, they're able to
select GENERIC_TIME accordingly and optimize out most of the timer
API.

Once the current timers have been reworked as proper clocksource
drivers, the rest of the place holders for the timer API can go
away and we can flip on GENERIC_TIME unconditionally.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>