Initial B&N Nook Color (encore) support

Bare-bones board file, comes with serial console, gpio keys,
MMC/SDCard and USB (peripheral) support.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
[tony@atomide.com: updated for irq and timer cleanup and to select the board by default]
Signed-off-by: Tony Lindgren <tony@atomide.com>