MGS-2842 ARM: imx: correct PFD setting rate flow

MGS-2842 ARM: imx: correct PFD setting rate flow

According to design, PFD needs to be gated before
setting rate, this patch adds warning for PFD when
there is any try to set PFD rate with gate open;

Since PFD may be enabled during kernel boot up,
here doing enable and disable before setting APLL_PFD2
rate is to make sure it is gated by clock framework
before setting rate.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
(cherry picked from commit bc731e14dc8401efa55fee65948c3ec31c9e5483)