Earlier BeagleBoards were using pad AH8 muxed to GPIO29 for MMC write-protect.
However, this signal has been changed to pad AG9 in board revision C2.

Fix this by adding mux configuration for pad AG9, runtime check for board
revisions and set the gpio number and pad muxing accordingly.
Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>