pwm: fsl-ftm: Added a dedicated IP interface clock

The current driver assumes that the ftm_sys clock works as one of the
clock sources for the IP block as well as the IP interface clock. This
assumption does not apply any more on the latest i.MX8x SoC family. On
i.MX8x SoCs, a dedicated IP interface clock is introduced and it must be
enabled before accessing any FTM registers. Moreover, the clock can not
be used as the source clock for the FTM IP block. This patch introduces
the ipg_clk as the dedicated IP interface clock and by default it is the
same as the ftm_sys clock if not specified.

Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>