ARM i.MX6: correct MLB clock configuration

According to the i.MX6 Dual/Quad technical reference manual
(Figure 18-2. Clock Tree - Part 1) the MLB clock is directly
feed by the AXI_CLK_ROOT. This is called 'axi' in our code.

Note that the clock of the MLB IP block on the i.MX6 is completely
independent of the PLL8 (MLB PLL). The MLB PLL isn't responsible
for feeding the MLB IP block with a clock. Instead, it's used
internally by the MLB module to sync the bus clock in case the MLB
6-pin interface is enabled:

MediaLB Control 0 Register, MLB150_MLBC0[5], MLBPEN:
1 MediaLB 6-pin interface enabled. MLB PLL and MLB PHY is enabled in this case.

I.e. the PLL8 MLB PLL has to be handled by the MLB driver and isn't needed
for clocking the MLB module itself.

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