stm32mp1: support forced boot mode

stm32mp1: support forced boot mode

The boot mode can be forced by key press
or by TAMP register, requested in kernel by syscon-reboot-mode

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>;
	};
};

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>