MLK-20886-2 imx: Add Virtual service glue layer

MLK-20886-2 imx: Add Virtual service glue layer

We use a glue layer to link the low level MU driver and virtual drivers.
This glue layer is named to virtual service (iMX VService). Virtual service
provides unified interfaces for setup connection with M4, get message buffer
and send/receive message, etc.

Multiple virtual drivers (i2c, gpio, etc)
             |
         iMX Vservice
             |
       imx_mu_m4 driver

For each virtual device, by default, the Vservice uses the device node property
"fsl,vservice-mu" to specify the MU node handler. A override function is also provided,
so te ARCH level can define its rule. We will use the override function for dynamically
select MU on 8QM/QXP.

Signed-off-by: Ye Li <ye.li@nxp.com>