ARM: dts: imx51-zii-rdu1: Do not specify "power-gpio" for hpa1

TPA6130A2 SD pin on RDU1 is not really controlled by SoC and instead
is only meant to notify the system that audio was "muted" by external
actors. To accommodate that, drop "power-gpio" property of hpa1 node as
well as specify a name for that GPIO so that userspace can access it.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>