davinci: support changing the clock rate in clock framework

clk_round_rate, clk_set_rate have been updated to handle dynamic
frequency changes.

The motivation behind the changes is to support dynamic CPU frequency
change.

davinci_set_pllrate() changes the PLL rate of a given PLL. This function
has been presented as a generic function though it has been tested only
on OMAP-L138 EVM. No other currently available DaVinci device will probably
use this function, but any future device specific changes will hopefully be
small enough to get taken care using a cpu_is_xxx() macro.

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