clk: imx25: Remove osc clock from driver

The 'osc' clock is already initialized by the fixed clock defined in
imx25.dtsi. The imx25 clock driver tries to add this clock for a second
time and fails with -EEXIST:
	i.MX clk 1: register failed with -17

As the clock is already properly setup in DT with a different driver, we
can completely remove the handling in the imx25 clock driver.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>