board: ge: bx50v3: Enable DM PWM for backlight

Add backlight and panel devicetree definitions
Use UCLASS_PANEL to enable backlight via display enable handler
Remove old explicit gpio code for handling backlight
Use cls command to initiate display in HW agnostic manner
Enable DM regulator and pwm

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>