stm32mp157c-ev1.dts: Fix optee status to 'okay'

The optee device status is disabled by default, change its status to 'okay'
in the dts of the EV1 board
Signed-off-by: NTimothée Cercueil <timothee.cercueil@st.com>

arch/arm/boot/dts/stm32mp157c-ev1.dts

@@ -375,3 +375,7 @@ &usbotg_hs {
 &usbphyc {
 	status = "okay";
 };
+
+&optee {
+	status = "okay";
+};