Blackfin: BF51x/BF52x: support GPIO Hysteresis/Schmitt Trigger options

Newer parts have optional Hysteresis/Schmitt Trigger options to help with
dirty signals.  So add some kconfig options for tuning this and enable it
by default for people.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>