mfd: cgbc: Add support for backlight

The Board Controller has control for display backlight.
Add backlight cell for the cgbc-backlight driver which
adds support for backlight brightness control.

Signed-off-by: Petri Karhula <petri.karhula@novatron.fi>
Reviewed-by: Thomas Richard <thomas.richard@bootlin.com>
Link: https://patch.msgid.link/20251205-cgbc-backlight-v6-2-e4175b0bf406@novatron.fi
Signed-off-by: Lee Jones <lee@kernel.org>

Author: petri-k

drivers/mfd/cgbc-core.c

@@ -237,6 +237,7 @@ static struct mfd_cell cgbc_devs[] = {
 	{ .name = "cgbc-i2c", .id = 1 },
 	{ .name = "cgbc-i2c", .id = 2 },
 	{ .name = "cgbc-hwmon"	},
+	{ .name = "cgbc-backlight" },
 };
 
 static int cgbc_map(struct cgbc_device_data *cgbc)