ARM: dts: Add devicetree for Gumstix Pepper board

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 <ashcharles@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>