drivers: pwm: add instance versions of the PWM_DT_SPEC* helpers
drivers: pwm: add instance versions of the PWM_DT_SPEC* helpers

The instance based version macros were not added when the new
PWM_DT_SPEC* helpers were introduced. This means drivers are forced to
make usage of DT_DRV_INST(inst) macro to obtain a node identifier, what
is required by existing macros. Following other driver classes, provide
the PWM_DT_SPEC_INST* family of macros.

Signed-off-by: Gerard Marull-Paretas <bedb45f43a286e49f4dcda039a436b216ae61ef3@nordicsemi.no>

zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr