xdma(4): Fix invalid pointer use (breaks arm.SOCFPGA build)

In xdma_handle_mem_node(), vmem_size_t and vmem_addr_t pointers were passed to
an FDT API that emits u_long values to the output parameter pointer. This
broke on systems with both xdma and 32-bit vmem size/addr types (SOCFPGA).

Reported by: tinderbox
Sponsored by: Dell EMC Isilon