Bluetooth: btnxpuart: Update firmware names

This updates the firmware names of 3 chipsets: w8987, w8997, w9098.
These changes are been done to standardize chip specific firmware
file names to be in sync with firmware names of newer chipsets.

The naming convention for BT-only files would be as follows:
For dual-radio WiFi+BT chipsets:
- <protocol-BT><chip-name>_bt_v<HW-version>.bin
For tri-radio WiFi+BT+15.4 chipsets:
- <protocol-BT><protocol-15.4><chip-name>_bt_v<HW-version>.bin

To maintain backward compatibility, this commit adds a provision to
request older firmware file name, if new firmware file name not found in
/lib/firmware/nxp/.

A new device tree property has been introduced called firmware-name, to
override the hardcoded firmware names (old and new) in the driver.

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