添加了基于dma传输的pwm输入脉宽测量驱动；目前支持N32G457；已经过测试

Author: chejia12

bsp/n32/libraries/n32_drivers/SConscript

@@ -54,8 +54,11 @@ if GetDepend(['BSP_USING_TIM_ETR_CNT']):
     src += ['tim_etr_cnt.c']     
 
 if GetDepend(['BSP_USING_TIM_EXT_COUNTER']):
-    src += ['drv_tim_external_counter.c']   
-    
+    src += ['drv_tim_external_counter.c']  
+
+if GetDepend(['BSP_USING_TIM_PULSE_WIDTH']):
+    src += ['drv_tim_pulse_width.c']    
+      
 path =  [cwd]
 path += [cwd + '/config']