rpmsg: virtio: allocate buffer from parent

Remoteproc is now capable to create one specific sub-device per
virtio link to associate a dedicated memory pool.
This implies to change device used by virtio_rpmsg for
buffer allocation from grand-parent to parent.

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>