davinci: support re-parenting a clock in the clock framework

The clk_set_parent() API is implemented to enable re-parenting
clocks in the clock tree.

This is useful in DVFS and helps by shifting clocks to an asynchronous
domain where supported by hardware

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