omap3: overo: Set the Arbor43C 'expansionname'

The Gumstix Arbor43C expansion board [1] uses a customized device tree
file.  When this expansion board is detected, load this file.

[1] https://store.gumstix.com/index.php/products/635/



Signed-off-by: Ash Charles <ashcharles@gmail.com>