drivers: pwm: mcux_ftm: use DT_INST_* defines instead of dts fixups

Convert the NXP MCUX FTM driver to use the DT_INST_* defines instead
of a mix of CONFIG_PWM_* and dts fixups. This simplifies adding new
device tree properties to the ftm nodes.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>