bus: mhi: Fix MHI DMA structure endianness

The MHI driver does not work on big endian architectures.  The
controller never transitions into mission mode.  This appears to be due
to the modem device expecting the various contexts and transfer rings to
have fields in little endian order in memory, but the driver constructs
them in native endianness.

Fix MHI event, channel and command contexts and TRE handling macros to
use explicit conversion to little endian.  Mark fields in relevant
structures as little endian to document this requirement.

Fixes: a6e2e352 ("bus: mhi: core: Add support for PM state transitions")
Fixes: 6cd330ae ("bus: mhi: core: Add support for ringing channel/event ring doorbells")
Signed-off-by: Paul Davey <paul.davey@alliedtelesis.co.nz>
Cc: stable@vger.kernel.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>