rockchip: bootrom: add definitions to retrieve BROM boot-source

The Rockchip BROM allows reading where it booted from from SRAM.
This adds the necessary definitions (as received from Kever) for
the location of this information in the RK3399's SRAM and naming
for the constants used.

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