firmware: qcom_scm: Expose qcom_scm_set_download_mode

Add support for other drivers to manage download mode by exposing
qcom_scm_set_download_mode (such as in commit: "power: reset:
msm-poweroff: Manage download mode"). Also, change the function to
support changing to different download modes with enum
qcom_download_mode, instead of only either "full dump mode" or "normal
restart".

Change-Id: Ic79d8dda97c17df70bd2cef5d9ce12ecb4c06d00
Signed-off-by: Elliot Berman <eberman@codeaurora.org>