drivers: pwm: mcux_ftm: allow to select clock source

FTM internal counter can be clocked by one of three clock sources
independent of the module bus clock. This patch introduces a DT property
to perform the clock selection from DT.

DT sources are updated to keep the current clock selection for all boards,
with exception of ucans32k1sic board which is migrated to use system
clock by default, as this seems to be a better choice for most cases.
Some PWM LED samples require slower clock so overlays are added for
those cases.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>