drivers: firmware: scmi: Introduce basic system power protocol

Add SCMI System Power Domain Protocol support with:

- System power state set operations
- Standard SCMI power states (shutdown, cold/warm reset, suspend)
- Graceful and forceful power transitions
- Optional NXP vendor-specific power states
- Device tree binding for "arm,scmi-system"

This enables system-wide power management through standardized
SCMI interfaces for reboot and power control operations.

Signed-off-by: Yongxu Wang <yongxu.wang@nxp.com>