See help for clk_get_rate(): "obtain the current clock rate (in Hz) for a
clock source. This is only valid once the clock source has been enabled."

It currently returns the correct value but that may not stay that way.
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>