rockchip: spl: support a 'same-as-spl'-specifier in the spl-boot-order

It is often desirable to configure the spl-boot-order (i.e. the order
that SPL probes devices to find the FIT image containing a full U-Boot)
such that it contains 'the same device the SPL stage was booted from'
early on.  To support this, we introduce the 'same-as-spl' specifier
for the spl-boot-order property.

This commit adds:
 - documentation for the new board_spl_was_booted_from() function that
   individual SoCs/boards should provide, if they can determine where
   the SPL was booted from
 - implements the new board_spl_was_booted_from() stub function
 - adds support for handling the 'same-as-spl' specifier and calling
   into the per-SoC/per-board support code.

This also updates the documentation for the 'u-boot,spl-boot-order'
property.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/device-tree-bindings/chosen.txt

@@ -56,10 +56,20 @@ Each list element of the property should specify a device to be probed
 in the order they are listed: references (i.e. implicit paths), a full
 path or an alias is expected for each entry.
 
+A special specifier "same-as-spl" can be used at any position in the
+boot-order to direct U-Boot to insert the device the SPL was booted
+from there.  Whether this is indeed inserted or silently ignored (if
+it is not supported on any given SoC/board or if the boot-device is
+not available to continue booting from) is implementation-defined.
+Note that if "same-as-spl" expands to an actual node for a given
+board, the corresponding node may appear multiple times in the
+boot-order (as there currently exists no mechanism to suppress
+duplicates from the list).
+
 Example
 -------
 / {
 	chosen {
-		u-boot,spl-boot-order = &sdmmc, "/sdhci@fe330000";
+		u-boot,spl-boot-order = "same-as-spl", &sdmmc, "/sdhci@fe330000";
 	};
 };