board: qualcomm: dragonboard820c: update readme

board: qualcomm: dragonboard820c: update readme

Update build instructions.

Be sure to use the u-boot-nodtb.bin image, as the Snapdragon platform
prioritizes the embedded Device Tree Blob (DTB) when present, rather
than the external one. The external DTB—modified by LK—is the version
required by the DB820c.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez@oss.qualcomm.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Christopher Obbard <christopher.obbard@linaro.org>
Reviewed-by:
Link: https://lore.kernel.org/r/20250407175617.3494506-1-jorge.ramirez@oss.qualcomm.com
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>