msm: restart: Add support for emergency download mode.

We now support a mechanism to put the device into emergency
download mode. Running 'reboot edl' will reset the device into
edl mode.

Change-Id: I8b966aeab1f7ceee9ea7e5e652a0145f38c14ea9
Signed-off-by: Ameya Thakur <ameyat@codeaurora.org>