rpmsg: Drop VIRTUALIZATION dependency from RPMSG_VIRTIO

A dependency to VIRTUALIZATION has been added to RPMSG_VIRTIO (back
when it was named RPMSG) in v3.10 kernel in commit 397944df
("rpmsg: fix kconfig dependencies for VIRTIO") to resolve Kconfig
warnings due to the inclusion of the virtio configuration file from
the ARM's KVM config file. The KVM config was fixed properly in the
subsequent release in commit 8bd4ffd6 ("ARM: kvm: don't include
drivers/virtio/Kconfig"). So, drop this unneeded VIRTUALIZATION
dependency from RPMSG_VIRTIO.

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