MLK-10007-02 arm: imx: enable pll1 when changing arm_podf value

MLK-10007-02 arm: imx: enable pll1 when changing arm_podf value

Before changing the arm_podf, the pll1 must be output enabled according
to the hardware design.

Refer to the clk tree implementation code for i.MX6SL. pll1's output will
be disabled when no clk is sourced from it. In busfreq code, in order to
successfully change the arm_clk rate, we must make sure pll1's output is
enabled before changing this clk rate. add imx6sl_enable_pll_arm() calls
to fulfill this requirement.

Additionally, only a bypassed output clk is ok, so no need to make sure pll1
is powered up.

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