ARM: imx: add necessary interface for pfd

Common clk framework will disable unused clks in late init only if
they are enabled by default and no one is using it, so we need to
add is_enabled callback for clk framework to get clks' status.

PFD clocks are enabled by hardware reset, so we need to add
interface for common clk framework to disable those unused ones for
saving power.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>