iwlwifi: mvm: optionally store D3 SRAM after resume

The D3 image SRAM is overwritten by the runtime image, so
it can't be accessed after resume. However, it can be very
useful to look at it to know what happened during D3, so
add the ability to store the image and make it available
in debugfs.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>