add memory barrier between set dma target address and enable dma

[wesleywesley]

in mrpc dma mode, dma target address register with offset 0x810-0x817 and mrpc dma mode enable register with offset 0x80c in gas space driver set the dma target address register first, then enable mrpc dma mode following, but because mrpc section with WC attribute, these 2 writes were combined and resulting enable dma first then followed by the dma target address, that's 3 dwords write from 0x80c to 0x817, add the necessory barrier between

the memory barrier also added when disable mrpc dma