clk: imx: gate2: Add cgr_mask for more flexible number of control bits

On some i.MX8 platforms, there are HW gates that share the same bit.
So in order to make this clock type more usable, use a mask to specify
how many bits belong to those HW gates.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>