bus: mhi: core: Add missing checks for MMIO register entries

As per documentation, fields marked as (required) in an MHI
controller structure need to be populated by the controller driver
before calling mhi_register_controller(). Ensure all required
pointers and non-zero fields are present in the controller before
proceeding with the registration.

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/1615315490-36017-1-git-send-email-bbhatt@codeaurora.org


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