rpmsg: virtio_rpmsg_bus: Switch to SPDX license identifier

Use the appropriate SPDX license identifier in the virtio rpmsg
bus driver source file and drop the previous boilerplate license
text.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>