davinci: dm6467/T EVM: fix setting up of reference clock rate

The DM6467 and DM6467T EVMs use different reference clock
frequencies. This difference is currently supported by having
the SoC code call a public board routine which sets up the reference
clock frequency. This does not scale as more boards are added.

Instead, use the clk_set_rate() API to setup the reference clock
frequency to a different value from the board file.

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