ARM: OMAP2+: clock: allow omap2_dpll_round_rate() to round to next-lowest rate

Change the behavior of omap2_dpll_round_rate() to round to either the
exact rate requested, or the next lowest rate that the clock is able to
provide.

This is not an ideal fix, but is intended to provide a relatively safe
way for drivers to set PLL rates, until a better solution can be
implemented.

For the time being, omap3_noncore_dpll_set_rate() is still allowed to
set its rate to something other than what the caller requested; but will
warn when this occurs.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Paul Walmsley <paul@pwsan.com>