[bsp] add n32 pwm driver

Author: chejia12

bsp/n32/n32gxx_lxx/libraries/n32_drivers/SConscript

@@ -45,6 +45,8 @@ if GetDepend(['RT_USING_RTC']):
 if GetDepend(['RT_USING_WDT']):
     src += ['drv_wdt.c']
 
+if GetDepend(['BSP_USING_PWM']):
+    src += ['drv_pwm.c']  
 path =  [cwd]
 path += [cwd + '/config']