ARM: dts: stm32: Enable reboot mode support to stm32mp157c

This patch enables syscon reboot mode.
Normal, fastboot, recovery, serialx and usb boot modes are
declared.
Boot mode value is stored in TAMP backup register (TAMP_BKP20R
offset 0x150).
system.d command is need to set the boot mode.
e.g.
$ systemctl reboot ums_mmc0

Change-Id: Ia0f376c1d32772c22d571559d445cfdf4557989e
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Reviewed-on: https://gerrit.st.com/109570
Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Alexandre TORGUE <alexandre.torgue@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/linux-stm32/+/144217
Tested-by: Alexandre TORGUE <alexandre.torgue@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/linux-stm32/+/146677

Author: patrickdelaunay

arch/arm/boot/dts/stm32mp157c.dtsi

@@ -1534,6 +1534,24 @@
 			#size-cells = <0>;
 			status = "disabled";
 		};
+
+		tamp: tamp@5c00a000 {
+			compatible = "simple-bus", "syscon", "simple-mfd";
+			reg = <0x5c00a000 0x400>;
+
+			reboot-mode {
+				compatible = "syscon-reboot-mode";
+				offset = <0x150>; /* reg20 */
+				mask = <0xff>;
+				mode-normal = <0>;
+				mode-fastboot = <0x1>;
+				mode-recovery = <0x2>;
+				mode-stm32cubeprogrammer = <0x3>;
+				mode-ums_mmc0 = <0x10>;
+				mode-ums_mmc1 = <0x11>;
+				mode-ums_mmc2 = <0x12>;
+			};
+		};
 	};
 
 	mlahb {