mhi: core: add base address field for MHI controller

Add a base address field in the MHI controller structure for
future use cases.

CRs-Fixed: 2341097
Change-Id: I2d90a32af91bde4c7553205d3f856d19dc138802
Acked-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>