drivers: bluetooth: hci: Add BT_HAS_HCI_VS selection for RPMSG.

Add explicit selection of the BT_HAS_HCI_VS for
the BT_RPMSG HCI driver. When the HCI over RPMSG
is used this dependency will not be solved automatically.
It is required to handle correctly Bluetooth identity
initialization.

Signed-off-by: Kamil Gawor <Kamil.Gawor@nordicsemi.no>