Gate off ENFC clock before setting clock rate

[Dmitry Lavnikevich]

Since last revision hardcoded manipulations with clock registers was
replaced with clk_disable/clk_enable calls.

For this purpose 2-bit gate clocks support was implemented for imx platform
(like it is done in current mainline kernel code) and enfc_gate 2-bit gate
clock was added in clk-imx.