remoteproc: copy parent dma_pfn_offset for vdev

When preparing the subdevice for the vdev, also copy dma_pfn_offset
since this is used for sub device dma allocations. Without that, there
is incoherency between the parent dma settings and the childs one,
potentially leading to dma_alloc_coherent failure (due to phys_to_dma
using dma_pfn_offset for translation).

Fixes: 086d0872 ("remoteproc: create vdev subdevice with specific dma memory pool")
Signed-off-by: Clement Leger <cleger@kalray.eu>
Acked-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>