bus: mhi: ep: Add support for managing MMIO registers

Add support for managing the Memory Mapped Input Output (MMIO) registers
of the MHI bus. All MHI operations are carried out using the MMIO registers
by both host and the endpoint device.

The MMIO registers reside inside the endpoint device memory (fixed
location based on the platform) and the address is passed by the MHI EP
controller driver during its registration.

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