media: venus: firmware: Leave a clue about obtaining CP VARs

The qcom_scm_mem_protect_video_var accepts two sets of <addr size>
pairs as arguments. They describe the virtual address ranges of the
CP (Content Protection) and CP_NONPIXEL regions. It is however not
immediately obvious how to obtain these values.

Leave a comment explaining how one can translate the vendor device
tree properties for use with the mainline driver.

Reviewed-by: Dikshita Agarwal <quic_dikshita@quicinc.com>
Reviewed-by: Vikash Garodia <quic_vgarodia@quicinc.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>