omap2 clock: rename, add comment to omap2_mpu_recalc()

omap2_mpu_recalc() actually just looks up what the MPU speed 'should'
currently be from the current virt_prcm_set rate set, rather than reporting
the actual hardware settings.  Clarify this by renaming it to
omap2_table_mpu_recalc() and adding some comments.

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