MLK-13279 rpmsg: imx: make vring address configurable by dts

MLK-13279 rpmsg: imx: make vring address configurable by dts

vring memory address was hardcoded at the top of the 1GB RAM.
For systems with a memory map with less or different than 1GB,
the hardcoded value might be not correct and cause issues.

This patch add the support to pass the vring address from device
tree configuration on the reg platform argument in the following
format:

    reg = <vring_address vring_size>

For example, for a 512MB system, with the rpmgs vring placed at
top of the memory the configuration will look like below:

    &rpmsg{
           reg = <0x9FFF0000 0x8000>;
    };

Signed-off-by: Juan Gutierrez <juan.gutierrez@nxp.com>