ARM: mach-shmobile: mackerel: clarify shdi/mmcif switch settings

* Pins 2 and 4 of switch 33 are documented as don't care on the PCB,
  my testing seems to confirm this.

* I have been unable to do anything sensible with S1 set to on.
  Am I missing something with regards to MMC1?

* Clarify which driver is needed for each switch setting.

* Should the AP4 board code be updated to allow the
  SHDI driver to access SHDI1 as the mackerel code does?

Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: Yusuke Goda <yusuke.goda.sx@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>