davinci: da850: add a .set_rate method to ref_clk

This patch allows setting the input clock frequency of the SoC from
the board specific code using the davinci_set_refclk_rate function.

Suggested-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>