ARM: dts: imx6qdl-udoo: disable AC'97 input pins pad drivers

AC'97 interface RXD and TXC pins are only used as SoC inputs, let's disable
pad drivers for them so we will be protected if, for example, TCLKDIR is
set by mistake in AUDMUX and causes TXC pin to be configured as an output.

This also changes pull direction on these pins from pull-up to pull-down
to match what the board AC'97 CODEC chip (VT1613) has on these pins.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>