rpmsg: Allow RPMSG_VIRTIO to be enabled via menuconfig or defconfig

Currently, RPMSG_VIRTIO can only be enabled if some other kconfig
option selects it. This does not allow it to be enabled for
virtualized systems where Virtio RPMSG is available over Virtio
MMIO or PCI transport.

This patch updates RPMSG_VIRTIO kconfig option so that we can
enable the VirtIO RPMSG driver via menuconfig or defconfig. The
patch also removes "select RPMSG_VIRTIO" from various remoteproc
kconfig options because it is now user selectable.

Signed-off-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>