Function write_repeated 

pub unsafe fn write_repeated<'a, C: Channel, W: Word>(
    ch: Peri<'a, C>,
    to: *mut W,
    len: usize,
    dreq: TreqSel,
) -> Transfer<'a, C> 

DMA repeated write.

SAFETY: Slice must point to a valid location reachable by DMA.