doc: board/qualcomm: Update RDP signing instructions

doc: board/qualcomm: Update RDP signing instructions

IPQ5424 expects U-Boot to have MBN version 7 header. The qtestsign tool
presently documented doesn't support v7. Hence update the instructions
to use a different tool that can support both v6 and v7.

Signed-off-by: Varadarajan Narayanan <quic_varada@quicinc.com>
Reviewed-by: Casey Connolly <casey.connolly@linaro.org>
Link: https://lore.kernel.org/r/20250304110105.2762124-4-quic_varada@quicinc.com
Signed-off-by: Casey Connolly <casey.connolly@linaro.org>