OMAP: PM constraints: add omap_pm_set_min_clk_rate()

Add omap_pm_set_min_clk_rate().  This constraint is meant for use by
device drivers to translate a certain device-specific performance
constraint (e.g., "minimum polygons per second") to a clock rate for
the driver's device, given the driver's intimate knowledge of the
device hardware (e.g., device type, device hardware revision, firmware
revision, etc.)  From a general PM core perspective, clock rate is
probably the closest general analog to "performance" that is
available, but the exact mapping from a use-case-specific performance
constraint to clock rate must be done by the driver.  Drivers intended for
upstream merging shouldn't hardcode specific clock rates in their code
without basing those rates on some performance criteria requested through
the driver's subsystem (ideally, from userspace).

Imre Deak <imre.deak@nokia.com> described the need and use-case for
this constraint, and discussed the implementation - thanks, Imre.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Imre Deak <imre.deak@nokia.com>