MLK-11813 change the clock type of 'pre_periph_sel' on imx6sl

The 'pre_periph2_sel' and 'pre_periph_clk' and other two clks
should have the ability to change its parent clk without disabling
itself first, so change the clk type of these clocks to make the
parent can be swithed without disabling the clock.

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