ARM: dts: vf610-twr: remove useless property for sound card.

This was added by:
Commit 8128c4f3 ("ARM: dts: vf610-twr: Add simple-card support.")

This useless property may cause some confusions for users.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>