clk: imx: gate2: Remove the IMX_CLK_GATE2_SINGLE_BIT special case

This was a hack which would allow multiple HW gates to be controlled
by a single bit. The only user of this is the imx_dev_clk_hw_gate_shared
which is not used anywhere as of now. Basically, complicates the logic
of the driver for no reason.

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>