mainboard/: Select MISSING_BOARD_RESET appropriately

We didn't have a hard_reset() implementation for these boards. So
select the board_reset() stub for them.

Change-Id: I77651e3844632fb1a347008c96e53d23cc5a2646
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/29170
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/sifive/hifive-unleashed/Kconfig

@@ -17,6 +17,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	def_bool y
 	select SOC_SIFIVE_FU540
 	select BOARD_ROMSIZE_KB_32768
+	select MISSING_BOARD_RESET
 
 config MAINBOARD_DIR
 	string