doc: stm32mp1: add page for device tree bindings

doc: stm32mp1: add page for device tree bindings

With device tree binding migration to yaml it is difficult to synchronize
the binding from Linux kernel to U-Boot.

Instead of maintaining the same dt bindings, this patch adds in the U-Boot
documentation the path to the device tree bindings in Linux kernel for
STMicroelectronics devices, when they are used without modification.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Add links for referenced text files.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>