firmware: imx: scu-irq: Remove unused export of imx_scu_enable_general_irq_channel

firmware: imx: scu-irq: Remove unused export of imx_scu_enable_general_irq_channel

Since its introduction, this symbol has not been used by any loadable
modules. It remains only referenced within imx-scu.c, which is always built
together with imx-scu-irq.c

As such, exporting imx_scu_enable_general_irq_channel is unnecessary, so
remove the export.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>