dt-bindings: net: ti,cc1352p7: Add bootloader-backdoor-gpios

bootloader-backdoor-gpio (along with reset-gpio) is used to enable
bootloader backdoor for flashing new firmware.

The pin and pin level to enable bootloader backdoor is configured using
the following CCFG variables in cc1352p7:
- SET_CCFG_BL_CONFIG_BL_PIN_NO
- SET_CCFG_BL_CONFIG_BL_LEVEL

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Ayush Singh <ayush@beagleboard.org>