ARM: imx: disable pll8_mlb in mx6q_clks

The MLB PLL clock's operation doesn't fit for clock framework and
it should be handled internally in MLB driver.
Remove initialization of pll8_mlb clock device but leave its
declaration in mx6q_clks to avoid affecting imx6q clock numbering.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
CC: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>