mailbox: mailbox-test: add support for separate tx/rx buffer with single channel

This patch adds support for different MMIO region for Tx and Rx paths.
If only one region is specified, it's assumed to be shared between Rx
and Tx, thereby retaining backward compatibility.

Also in order to support single channel dealing with both Tx and Rx with
dedicated MMIO regions, Tx channel itself is assigned to Rx if MMIO
regions are different and Rx is not specified.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>