power: reset: Add qcom-dload-mode driver

Add a driver to manage download mode for Qualcomm SoCs. The driver can
force SoC to restart into a dump collection mode when panicking for
post-morten analysis, as controlled by
CONFIG_POWER_RESET_MSM_DOWNLOAD_MODE_DEFAULT. The driver also can reboot
into "emergency download mode," which restarts SoC into mode which
permits re-flashing storage via Qualcomm Sahara protocol.

Change-Id: I30c83bf020715b7a80c00faa19280aec7f708435
Signed-off-by: Elliot Berman <eberman@codeaurora.org>