[bsp/nrf5x]: add the driver of pwm

Author: supperthomas

bsp/nrf5x/libraries/drivers/SConscript

@@ -19,8 +19,12 @@ if GetDepend(['BSP_USING_QSPI_FLASH']):
 
 if GetDepend(['BSP_USING_SPI']):
     src += ['drv_spi.c']
+    
 if GetDepend(['BSP_USING_GPIO']):
     src += ['drv_gpio.c']
+    
+if GetDepend(['BSP_USING_PWM']):
+    src += ['drv_pwm.c']
 
 path =  [cwd]