MLK-10007-03 arm: imx: fix ARM and IPG clk ratio in low_bus_freq_mode

On i.MX6SL, we must make sure ARM:IPG clock ratio is within 12:5 when entering
wait mode. If the system is in low_bus_freq_mode, the IPG is at 12MHz
according the busfreq code. So the max rate of ARM is 28.8MHz when entering
wait mode. As there is no way run at this clk rate, so set ARM to run from
24MHz OSC.

Signed-off-by: Bai Ping <b51503@freescale.com>