ARM: dts: imx: rdu1: cosmetic changes to the audio nodes

This renames the audio device to "Front" in order to match the RDU2
naming, which makes it easier for a unified userspace to deal with
both boards.

Also rename the phandle for the headphone amplifier to more closely
resemble the RDU2 DTS.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>