Some controllers can opt to not have MHI download a firmware
image to have the device bootup and can find the device in a
pass through execution environment, ready to go. Thus, MHI
controllers for those devices do not need fw_image defined.
Make it optional to accommodate different bootup modes.
Suggested-by: Loic Poulain <loic.poulain@xxxxxxxxxx>
Signed-off-by: Bhaumik Bhatt <bbhatt@xxxxxxxxxxxxxx>