am335x: pepper: Add Gumstix Pepper AM335x-based machine

This adds the Gumstix Pepper[1] single-board computer based on the
TI AM335x processor. Schematics are available [2].

[1] https://store.gumstix.com/index.php/products/344/
[2] https://pubs.gumstix.com/boards/PEPPER/



Signed-off-by: Ash Charles <ash@gumstix.com>
[trini: Move 'cdev' in board.c down to under #ifdef's where it's used]
Signed-off-by: Tom Rini <trini@ti.com>