firmware: imx: scu-irq: add imx_scu_irq_get_status

Extract the scu irq get status code from imx_scu_irq_work_handler and
make into a new function imx_scu_irq_get_status which could be used
by others, such as SECO.

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