ARM: imx6q: clk: Change hsi_tx clock to be 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.  So, this patch changes the
hsi_tx clock to be 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>