ARM: davinci: da850: use clk->set_parent for async3

The da850 family of processors has an async3 clock domain that can be
muxed to either pll0_sysclk2 or pll1_sysclk2. Now that the davinci clocks
have a set_parent callback, we can use this to control the async3 mux
instead of a stand-alone function.

This adds a new async3_clk and sets the appropriate child clocks. The
default is use to pll1_sysclk2 since it is not affected by processor
frequency scaling.

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