davinci: make clock rate re-calculation easy

Make clock rate recalculation easy by having a re-calculate
function for each clock.

The existing functions for calculation of output rates of PLL
and PLL-derived sysclks have been convered to the new
re-calculate API.

A new function is introduced to take care of rate
(re)calculation for leaf clocks.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>