doc: board/qualcomm: document RDP building/flashing

Introducing basic support for Qualcomm IPQxxx based RDPs.
Document the build and flashing steps.

Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
Signed-off-by: Varadarajan Narayanan <quic_varada@quicinc.com>
Link: https://lore.kernel.org/r/20250226064505.1178054-2-quic_varada@quicinc.com
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
2 files changed