otPlatMcuPowerState enum

NCP's MCU stays on and active all the time. When the NCP's desired power state is set to `ON`, host can send messages to NCP without requiring any "poke" or external triggers.

NCP's MCU can enter low-power (energy-saving) state. When the NCP's desired power state is set to `LOW_POWER`, host is expected to "poke" the NCP (e.g., an external trigger like an interrupt) before it can communicate with the NCP (send a message to the NCP). The "poke" mechanism is determined by the platform code (based on NCP's interface to the host). While power state is set to `LOW_POWER`, NCP can still (at any time) send messages to host. Note that receiving a message from the NCP does NOT indicate that the NCP's power state has changed, i.e., host is expected to continue to "poke" when it wants to talk to the NCP until the power state is explicitly changed (by a successful call to `otPlatSetMcuPowerState()` changing the state to `ON`).

NCP is fully off. An NCP hardware reset (via a RESET pin) is required to bring the NCP back to `SPINEL_MCU_POWER_STATE_ON`. RAM is not retained after reset.