OMAP clock: make the fixed divisor clock code available for all OMAPs

One of the OMAP1 clocks can use the fixed divisor recalculation code
introduced in the OMAP2 clock code, so rename the
omap2_fixed_divisor_recalc() function to omap_fixed_divisor_recalc()
and make it available to all OMAPs.  A followup patch converts the OMAP1
clock.

Signed-off-by: Paul Walmsley <paul@pwsan.com>