board: ge: bx50v3: Disable unused pins

Certain pins are not used on the i.MX6, and should have a neutral
pad configuration in order to reduce electrical interference on
the board. This commit defines these pins with a default value
rather than relying on the system defaults.

Signed-off-by: Justin Waters <justin.waters@timesys.com>
Signed-off-by: Akshay Bhat <akshay.bhat@timesys.com>
Cc: Stefano Babic <sbabic@denx.de>