davinci: clock: Check CLK_PSC flag before disabling PSC

Some modules do not have PSC to control their clocks.
The 'lpsc' field in the clk structure is 0 for such clocks.

In the clock disable function check for CLK PSC flag before
disabling the PSC. If this is not taken care of then it may
so happen that module controlled by LPSC 0 is erroneously disabled.

Signed-off-by: Chaithrika U S <chaithrika@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>