ARM: dts: imx51-zii-scu3-esb: Add switch IRQ line pinumx config

Instead of relying on default values, configure PAD_AUD3_BB_CK to be a
GPIO explicitly. While at, it change the pad configuration to enable
a 100K pull-down (the pin is used as IRQ_TYPE_LEVEL_HIGH).

Cc: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: cphealy@gmail.com
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>