gpio/mxc: use the edge_sel feature if available

Some mxc processors have an edge_sel feature, which allows the IRQ to be
triggered by any edge.

This patch makes use of this feature if available, which skips mxc_flip_edge().

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Acked-by: Sascha Hauer <kernel@pengutronix.de>
Cc: <linux-arm-kernel@lists.infradead.org>
Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>