Patch from: Peter Teichmann, edited by Russell King.

* a bugfix to the pseudo palette handling in 16/32 bit modes: the
  pseudo palette is always kept in 32 bit integers
* a bugfix to setcolreg() in 16 bit modes: there have to be 32 colour
  registers (this has nothing to do with the 16 pseudo palette
  colours, it is a bit misleading that setcolreg() is used both for
  pseudo palette setup and DIRECTCOLOR colour translation)
  (The behaviour should be selected using the visual and the rgb
   representation rather than the current BPP setting. --rmk)
* change 16 bit visual from TRUECOLOR to DIRECTCOLOR
* take the limited bandwidth of the RiscPC video memory into account
  for acornfb_valid_pixrate()
* acornfb_vidc20_find_rates() now takes into account the differences
  between VRAM and DRAM when setting the FIFO preload value. With
  VRAM, the FIFO can be set to the highest possible setting because
  there are no latency considerations for other memory accesses.
  However, in 64 bit bus mode the FIFO preload value must not be set
  to VIDC20_CTRL_FIFO_28 because this will let the FIFO overflow.
  See VIDC20 manual page 33 (6.0 Setting the FIFO preload value).

* select Multifreq monitor as default, increase frequency range for
  Multifreq monitor (this is of course a philosophical question, but
  I believe a sensible default setting should not impose too much
  limits to the user)
  (I dropped this change because it is known that some monitor
   hardware shipped with Acorn machines does not take kindly to
   wrong timing signals.  If people really care, they should use
   the kernel command line to select the correct monitor type. --rmk)