On msm8916 and msm8996 boards a secure io-write is used to write the
magic for selecting "download mode", specify this address in the
DeviceTree.

Note that qcom_scm.download_mode=1 must be specified on the kernel
command line for the kernel to attempt selecting download mode.
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>