spl: document 'chosen/u-boot, spl-boot-device'

To let the full U-Boot know where it was booted from (i.e. which of
the entries in /chosen/u-boot,spl-boot-order' contained a valid
image), we define (and document) /chosen/u-boot,spl-boot-device as the
property that could/should automatically be injected by SPL.

This commit only contains a documentation change, which documents the
new property and the intended usage.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>

doc/device-tree-bindings/chosen.txt

@@ -73,3 +73,13 @@ Example
 		u-boot,spl-boot-order = "same-as-spl", &sdmmc, "/sdhci@fe330000";
 	};
 };
+
+u-boot,spl-boot-device property
+-------------------------------
+
+This property is a companion-property to the u-boot,spl-boot-order and
+will be injected automatically by the SPL stage to notify a later stage
+of where said later stage was booted from.
+
+You should not define this property yourself in the device-tree, as it
+may be overwritten without warning.