sk(4): Rework busdma(9) related bits

sk(4): Rework busdma(9) related bits

- According to data sheet, sk(4) does support 64bits address space.
  However, the RX/TX descriptors' high 32bits must be same, i.e.
  descriptors' can't spread above and below 4Gbytes boundary.  Since
  sk(4)'s RX/TX descriptors are in contiguous memory, this means we
  only need to make sure that the contiguous memory does not cross
  4Gbytes boundary.
- RX/TX descriptors' is 8bytes aligned not 64bytes aligned.
- Split RX/TX descriptor rings.
- Use bus_dmamem_coherent() to allocate RX/TX descriptor rings.
- Don't sync coherent memory.