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>

include/linux/mhi.h

@@ -296,7 +296,7 @@ struct mhi_controller_config {
  * @wake_db: MHI WAKE doorbell register address
  * @iova_start: IOMMU starting address for data (required)
  * @iova_stop: IOMMU stop address for data (required)
- * @fw_image: Firmware image name for normal booting (required)
+ * @fw_image: Firmware image name for normal booting (optional)
  * @edl_image: Firmware image name for emergency download mode (optional)
  * @rddm_size: RAM dump size that host should allocate for debugging purpose
  * @sbl_size: SBL image size downloaded through BHIe (optional)