Bluetooth: btnxpuart: Add support for IW615 chipset

This adds support for IW615 chipset with it's bootloader signature
and firmware file.

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