OMAP3: Beagle: Add board revision detection

OMAP3: Beagle: Add board revision detection

With BeagleBoard revision C some HW changes are introduced (e.g. PinMUX)
which might need different software handling. For this, GPIO pin 171 (GPIO
module 6, offset 11) can be used to check for board revision. If this pin
is low, we have a rev C board. Else it must be a revision Ax or Bx board.

To handle board differences you can call function beagle_get_revision().
E.g.:

if (beagle_get_revision()) {

/* do special revision C stuff here */

}

Signed-off-by: Dirk Behme <dirk.behme@googlemail.com>