gpio: port to am335x

 * port libgpio to am335x.
 * increase NR_IRQ to 16 to avoid too many IRQs requested error.
 * move gpio binary to /sbin (that's where mount looks for drivers).
 * fix bug that re-enabled banks that were disabled due to bad revision.
 * fix padconf masks for dm37xx in gpio driver (should be 16-bit).
 * correct a few typos in README.txt and comments.

Change-Id: I32374415a97135174c5224f76374a870909fa90c