stm32mp1: configs: add BACKLIGHT_GPIO support

Backlight of panel raydium RM68200 is controlled by a simple gpio,
thus we activate the support for the needed driver.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/stm32mp15_optee_defconfig b/configs/stm32mp15_optee_defconfig
index 0860a26..f480c3d 100644
--- a/configs/stm32mp15_optee_defconfig
+++ b/configs/stm32mp15_optee_defconfig
@@ -106,4 +106,5 @@
 CONFIG_USB_GADGET_PRODUCT_NUM=0x5720
 CONFIG_USB_GADGET_DWC2_OTG=y
 CONFIG_DM_VIDEO=y
+CONFIG_BACKLIGHT_GPIO=y
 CONFIG_FDT_FIXUP_PARTITIONS=y