ARM: dts: imx6: RIoTboard explicitly define pad settings

Instead of relying on defaults or bootloader settings, explicitly define
all pad settings.

This resolves reported issues of no analogue audio output.

Signed-off-by: Iain Paton <ipaton0@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>