clocksource: Deprecate clock string across the SH drivers.

We want to get rid of the clock string from platform data entirely,
depending on the clkdev-based clock lookup to do the right thing for us
instead.

This converts all of the SH drivers to request their associated function
clocks directly, and if there is no match for that then we fall back on
the legacy lookup while warning about it. After all of the outstanding
CPUs have been converted to clkdev lookups the clock string will be
killed off completely.

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