ARM: dts: imx6: Change the clock name for spba clock

Audio IP need the spba clock, but original clock name "dma" is not
accurate, so change it to name "spba". The audio driver has been
using the new name "spba", the binding document has been updated.

Signed-off-by: Shengjiu Wang <shengjiu.wang@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>