boards/hifive1: include cpu/fe310 features

The board should process its CPU features.
This has the consequence to make 'periph_pm' visible to the board.

I also removed the duplicate FEATURES_PROVIDED from the board defined in
the cpu.

boards/hifive1/Makefile.features

 # Put defined MCU peripherals here (in alphabetical order)
-FEATURES_PROVIDED += periph_cpuid
 FEATURES_PROVIDED += periph_gpio periph_gpio_irq
 #FEATURES_PROVIDED += periph_pwm
 FEATURES_PROVIDED += periph_rtc
@@ -14,3 +13,5 @@ endif
 
 # The board MPU family (used for grouping by the CI system)
 FEATURES_MCU_GROUP = risc_v
+
+include $(RIOTCPU)/fe310/Makefile.features