firmware: qcom_scm: Add a call for getting dload mode

Add an SCM call to read the dload mode cookie, which can
be used in various drivers to make decisions based on the
dload mode. Earlier, this support was not there and HLOS
was not aware of changes in this cookie, outside of its
domain.

Change-Id: I3bb82b65bc411354090b34b98f7e651dd1888e5b
Signed-off-by: Khaja Hussain Shaik Khaji <quic_kshaikkh@quicinc.com>