ARM: dts: imx6sl: add baud clock and clock-names for ssi

Baud clock is used for bit clock generation in master mode. Ipg clock
is peripheral clock and peripheral access clock.

Signed-off-by: Shengjiu Wang <shengjiu.wang@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>