MLK-13147: arm: imx6q: lpddr2 freq simplify system clock sources

MLK-13147: arm: imx6q: lpddr2 freq simplify system clock sources

Simplify system root clock sources, only use pll2_pfd2 and osc
for system clocks, when busfreq request high bus mode or audio bus
mode only update dividers to achieve operating frequencies from
same source (pll2_pfd2).

Bus freq mode      (400M)     (100M)     (24Mhz)
mmdc_ch0_clk_root   @396MHz    @99MHz      @24Mhz
axi_clk_root        @198MHz    @49.5MHz    @24Mhz
ahb_clk_root        @132MHz    @49.5Mhz    @24Mhz
ipg_clk_root        @66MHz     @24.75Mhz   @12Mhz

Signed-off-by: Adrian Alonso <adrian.alonso@nxp.com>