OMAP hardware doesn't provide a phyisical DTR line, but
some configurations may need a DTR line which tracks whether
the device is open or not.

So allow a gpio to be configured as the DTR line.
Signed-off-by: NNeilBrown <neilb@suse.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>