ARM: clk-imx6sl: refine clock tree for SSI

Each SSI has "ssi", "ssi_ipg" clocks, and they share same gate bits.

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

diff --cc arch/arm/mach-imx/clk-imx6sl.c
Simple merge

diff --cc include/dt-bindings/clock/imx6sl-clock.h
Simple merge