ipc: rpmsg_service: Remove virtual shared memory device

A virtual metal_device is created, next the needed IO regions are created
and added to this device. Immediately we extract these regions back out
and make use of them. There is no reason to create the metal_device and
add the IO regions to it, instead simply use the IO regions directly.

This is similar to what was already done to the openamp_rsc_table sample.

Signed-off-by: Andrew Davis <afd@ti.com>