display: ili9340: Make reset gpio definition optional.
display: ili9340: Make reset gpio definition optional.

ILI9340 may not need a reset signal pin for certain LCD shields.

Signed-off-by: Bernard Lee <c4b2740814effaea1093e2887ceed6ed5623afee@nordicsemi.no>

galak/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr