MLK-12868-03 ARM: imx: add busfreq support on imx6ull

MLK-12868-03 ARM: imx: add busfreq support on imx6ull

Add busfreq support on i.MX6ULL. per to the design
team, there is a 24MHz low power run mode on i.MX6ULL.
the define for this mode is as below:
----------------------------
|cpu    DRAM    AXI    AHB |
 24MHz  24MHz   24MHz  24MHz

The mode can be implemented as 'low_bus_mode' in busfreq,
compared to i.MX6UL, the additional code we need to add is
clk change for cpu core. so in low_bus_mode, the cpu will run
at 24MHz.

Signed-off-by: Bai Ping <ping.bai@nxp.com>