ARM: dts: pandaboard: fix pinmux for gpio user button of Pandaboard ES

The pinmux control register offset passed to OMAP4_IOPAD is odd.

Fixes: ab9a1366 ("ARM: dts: pandaboard: add gpio user button")
Cc: stable@vger.kernel.org
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>