Bluetooth: btnxpuart: Add GPIO support to power save feature

This adds support for driving the chip into sleep or wakeup with a GPIO.

If the device tree property device-wakeup-gpios is defined, the driver
utilizes this GPIO for controlling the chip's power save state, else it
uses the default UART-break method.

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>