clk: imx: add 'is_prepared' clk_ops callback for pllv3 clk

Add 'is_prepared' callback function for pllv3 type clk to make sure when
the system is bootup, the unused clk is in a known state to match the
prepare count info.

Signed-off-by: Bai Ping <b51503@freescale.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>