doc: Update console driver documentation
Added BeagleBoard BSP

Coding done in course of GSoC2012.

Commit edited to be brought up-to-date with mainline by
Ben Gras <beng@shrike-systems.com>.
BSP for several Beagle products

Specifically the beagleboard, beagleboard xM, beaglebone, beaglebone black.

More info on these targets: http://www.beagleboard.org/

    This commit forms a basic BSP by combining Claas's work with
        . new clock and irq code and definitions for
          beagle targets (beagleboard and beaglebones), mostly
          reused from the Minix codebase, thus making
          irqs, ticks and non-polled console mode work too
        . new timer code for ns timing with high timer resolution,
          24MHz on the AM335X and 13MHz on the DM37XX
        . select the console uart based on target at configure time
        . removing all the lpc32xx-specific macros and code and
          other unused code and definitions that the beagle bsp
          was based on
        . re-using some standard functions instead of lpc32xx versions
        . fixed some whitespace problem in preinstall.am
        . fixed some compile warnings
        . configure MMU: set 1MB sections directly in the TTBR,
	  just to show the difference between cacheable RAM and
	  non-cacheable device memory and invalid ranges; this lets us
	  turn on caches and not rely on boot loader MMU configuration.
	  Verified to work when MMU is initially either on or off when
	  RTEMS gets control.

    Signed-Off-By: Ben Gras <beng@shrike-systems.com>