omap2 clock: add fixed divisor clock code

The rates of some clocks are equal to their parents' rates, divided by some
fixed integer.  This contrasts with the existing 'followparent' clocks,
which follow their parents' rates strictly; and the existing 'clksel' clocks,
which follow their parents' rates divided by a runtime-selectable divisor.
Add code to implement these clocks without resorting to specifying a
fixed rate.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>