firmware: imx: enable imx scu general irq function

The System Controller Firmware (SCFW) controls RTC, thermal
and WDOG etc., these resources' interrupt function are managed
by SCU. When any IRQ pending, SCU will notify Linux via MU general
interrupt channel #3, and Linux kernel needs to call SCU APIs
to get IRQ status and notify each module to handle the interrupt.

Since there is no data transmission for SCU IRQ notification, so
doorbell mode is used for this MU channel, and SCU driver will
use notifier mechanism to broadcast to every module which registers
the SCU block notifier.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>