tmio_mmc: add bus_shift support

Some ASIC3 devices in the wild are connected with the address bus shifted
by one line, so that its 16-bit registers appear 32-bit aligned in host
memory space.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Acked-by: Ian Molton <ian@mnementh.co.uk>
Signed-off-by: Pierre Ossman <pierre@ossman.eu>