MLK-10161-2: ARM: imx6sl: Add SPDIF_GCLK clock in clock tree

MLK-10161-2: ARM: imx6sl: Add SPDIF_GCLK clock in clock tree

As spdif driver will register SPDIF clock to regmap, regmap will do
clk_prepare in init function, so SPDIF clock is prepared in probe, then its
root clock (pll clock) is prepared also, which cause the arm can't enter
low power mode.
Add SPDIF_GCLK in clock tree which share same gate bits with SPDIF clock.
Its root clock is ipg clock, and register it to regmap, then the issue can be
fixed.

Signed-off-by: Shengjiu Wang <shengjiu.wang@freescale.com>