bus: mhi: Make firmware image optional for controller

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@linaro.org>
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
Reviewed-by: Jeffrey Hugo <jhugo@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/1615315595-37750-1-git-send-email-bbhatt@codeaurora.org


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>