wandboard: Add board revision detection support

There are two revisions of wandboard: version B1 and C1.

Add the revision detection support, so that the correct dtb file can
be automatically loaded.

Based on the patch from Richard Hu <hakahu@gmail.com>.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Tested-By: Vagrant Cascadian <vagrant@aikidev.net>