Note: This patch implements in-band messaging protocol only.
ARM has launched a next version of MHU i.e. MHUv2 with its latest
subsystems. The main change is that the MHUv2 is now a distributed IP
with different peripheral views (registers) for the sender and receiver.

Another main difference is that MHUv1 duplex channels are now split into
simplex/half duplex in MHUv2. MHUv2 has a configurable number of
communication channels. There is a capability register (MSG_NO_CAP) to
find out how many channels are available in a system.

The register offsets have also changed for STAT, SET & CLEAR registers
from 0x0, 0x8 & 0x10 in MHUv1 to 0x0, 0xC & 0x8 in MHUv2 respectively.

0x0    0x4  0x8  0xC             0x1F
------------------------....-----
| STAT |    |    | SET |    |   |
------------------------....-----
      Transmit Channel

0x0    0x4  0x8   0xC            0x1F
------------------------....-----
| STAT |    | CLR |    |    |   |
------------------------....-----
        Receive Channel

The MHU controller can request the receiver to wake-up and once the
request is removed, the receiver may go back to sleep, but the MHU
itself does not actively put a receiver to sleep.

So, in order to wake-up the receiver when the sender wants to send data,
the sender has to set ACCESS_REQUEST register first in order to wake-up
receiver, state of which can be detected using ACCESS_READY register.
ACCESS_REQUEST has an offset of 0xF88 & ACCESS_READY has an offset
of 0xF8C and are accessible only on any sender channel.

This patch adds necessary changes in a new file required to support the
latest MHUv2 controller. This patch also needs an update in DT binding
for ARM MHUv2 as we need a second register base (tx base) which would
be used as the send channel base.

Change-Id: I1455e08b3d88671a191c558790c503eabe07a8e6
Signed-off-by: Tushar Khandelwal <tushar.khandelwal@arm.com>