backlight: Add Congatec Board Controller (CGBC) backlight support

backlight: Add Congatec Board Controller (CGBC) backlight support

This driver provides backlight brightness control through the Linux
backlight subsystem. It communicates with the board controller to
adjust LCD backlight using PWM signals. Communication is done
through Congatec Board Controller core driver.

Tested-by: Thomas Richard <thomas.richard@bootlin.com>
Reviewed-by: Thomas Richard <thomas.richard@bootlin.com>
Reviewed-by: Daniel Thompson (RISCstar) <danielt@kernel.org>
Signed-off-by: Petri Karhula <petri.karhula@novatron.fi>
Link: https://patch.msgid.link/20251205-cgbc-backlight-v6-1-e4175b0bf406@novatron.fi
Signed-off-by: Lee Jones <lee@kernel.org>