ARM: imx6q: clk: Add support for mipi_core_cfg clock as a shared clock gate

The CG8 field of the CCM CCGR3 register is named as 'mipi_core_cfg' clock,
according to the i.MX6q/sdl reference manuals.  This clock is actually the
gate for several clocks, including the hsi_tx_sel clock's output and the
video_27m clock's output.  The MIPI DSI host controller embedded in the
i.MX6q/sdl SoCs uses the video_27m clock to generate PLL reference clock and
MIPI core configuration clock.  In order to gate/ungate the two MIPI DSI
host controller relevant clocks, this patch adds the mipi_core_cfg clock as
a shared clock gate.

Suggested-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>