treewide: use full path to pwm.h header

The build infrastructure should not be adding the drivers subdirectory
to the include path.  Fix the legacy uses that depended on that
addition.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

drivers/pwm/pwm_mchp_xec.c

@@ -9,7 +9,7 @@
 #include <logging/log.h>
 LOG_MODULE_REGISTER(pwm_mchp_xec, CONFIG_PWM_LOG_LEVEL);
 
-#include <pwm.h>
+#include <drivers/pwm.h>
 #include <soc.h>
 #include <errno.h>
 

drivers/pwm/pwm_mcux.c

@@ -5,7 +5,7 @@
  */
 
 #include <errno.h>
-#include <pwm.h>
+#include <drivers/pwm.h>
 #include <soc.h>
 #include <fsl_pwm.h>
 #include <fsl_clock.h>