msm: restart: Add support for MSM8974

Refactor the existing restart code to split out the
download mode configuration into its own function, and add
support for restarting MSM8974 targets using the new
PS_HOLD register.

Change-Id: Ibe0677d1054769c94edec390d3e7b4e5dd877b79
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>