ARM: dts: msm: add imem dt entry for download mode for 8994

Add device tree entry for download mode imem base and size that
can be used by the msm poweroff driver to set the download mode
cookies.

Change-Id: I80de52dfae5ed78f8bb934f72f283b4122a771b3
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>