linux-imx

rpmsg: imx: add the initial imx rpmsg support

Based on "virtio_rpmsg_bus" driver, This patch-set is used to set up
the communication mechanism between A core and M core on i.MX AMP SOCs.

Add the initial imx rpmsg support glue driver and one pingpong demo,
demonstrated the data transactions between A core and remote M core.
Distributed framework is used in IMX RPMSG implementation, refer to the
following requirements:
  - The CAN functions contained in M core and RTOS should be ready and
    complete functional in 50ms after AMP system is turned on.
  - Partition reset. System wouldn't be stalled by the exceptions (e.x
    the reset triggered by the system hang) occurred at the other side.
    And the RPMSG mechanism should be recovered automactilly after the
    partition reset is completed.
In this scenario, the M core and RTOS would be kicked off by bootloader
firstly, then A core and Linux would be loaded later. Both M core/RTOS
and A core/Linux are running independly.

One physical memory region used to store the vring is mandatory required
to pre-reserved and well-knowned by both A core and M core

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>