ARM: dts: imx51: Improve SSI clocks description

SSI block has 'ipg' clock for internal peripheral access and also 'baud' clock
for generating bit clock when SSI operates in master mode.

Add the extra 'baud' clock so that we can have SSI functional in master mode.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>