rtc: imx-sc: add rtc alarm support

Add i.MX system controller RTC alarm support, the RTC alarm
is implemented via SIP(silicon provider) runtime service call
and ARM-Trusted-Firmware will communicate with system controller
via MU(message unit) IPC to set RTC alarm. When RTC alarm fires,
system controller will generate a common MU irq event and notify
system controller RTC driver to handle the irq event.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>