clk: imx: clk-gate2: allow custom gate configuration

The 2-bit gates found i.MX and Vybrid SoC support different clock
configuration:

0b00: clk disabled
0b01: clk enabled in RUN mode but disabled in WAIT and STOP mode
0b10: clk enabled in RUN, WAIT and STOP mode (only Vybrid)
0b11: clk enabled in RUN and WAIT mode

For some clocks, we might want to configure different behaviour,
e.g. a memory clock should be on even in STOP mode. Add a new
function imx_clk_gate2_cgr which allow to configure specific
gate values through the cgr_val parameter.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>