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

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>