davinci: clock: let clk->set_rate function sleep

When supporting I2C/SPI based on-board PLLs like CDCE949,
it is essential that clk->set_rate be able to sleep.

Currently, this is not possible because clk->set_rate is
called from within spin-lock in clk_set_rate

This patch brings clk->set_rate outside of the spin-lock
and lets the individual set_rate implementations achieve
serialization through appropiate means.

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