- Rename REG_DL to REG_DLL and REG_DLH.
- Always treat DLL and DLH as two separate 8-bit registers instead of one
  16-bit register.

Additionally, remove the probe for the high 4 bits of IER being 0 and don't
assume we can always read/write 0 to/from those bits.

These changes allow uart(4) to drive the UARTs on the Intel XScale PXA255.

Reviewed by:	marcel