SSD2119: make use of DMA

This is mostly a copy from Eddie's work posted here:
http://forum.chibios.org/phpbb/viewtopic.php?f=11&t=851#p11054
No work was done towards making it work as fast as possible.
Tested with:
https://github.com/etmatrix/ChibiOS-GFX-Example/blob/master/bench/main.c
Results show performance of ~5.34 Mpx/s with use of DMA compared to
~4.78 Mpx/s without.
Various readme update

For GDISP drivers SSD2119 & SSD1289.
SSD2119: GDISP_USE_FSMC "switch" added

Enable FSMC functions only when GDISP_USE_FSMC is set.
SSD2119: FSMC settings tuned

With FSMC BTR timing register settings tuned and DMA on, we get over
10.2 Mpx/s instead of 4.7 Mpx/s.