ARM: davinci: clk: add set_parent callback for mux clocks

Introduce a set_parent callback that will be used for mux clocks, such as
the USB PHY muxes and the async3 clock domain mux.

Signed-off-by: David Lechner <david@lechnology.com>
[nsekhar@ti.com: checkpatch fixes]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>