clk: imx: gate2: Allow single bit gating clock

Audiomix on i.MX8MP registers two gates that share the same enable count
but use the same bit to control the gate instead of two bits. By adding
the flag IMX_CLK_GATE2_SINGLE_BIT we allow the gate2 to use the generic
gate ops for enable, disable and is_enabled.
For the disable_unused, nothing happens if this flag is specified.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>