mb/sifive: Get rid of device_t

Use of device_t has been abandoned in ramstage.

Change-Id: I8c6358d072b25ab4758637da989883daa600c8ee
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/26099
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>

src/mainboard/sifive/hifive-unleashed/mainboard.c

@@ -17,7 +17,7 @@
 #include <bootblock_common.h>
 #include <timestamp.h>
 
-static void mainboard_enable(device_t dev)
+static void mainboard_enable(struct device *dev)
 {
 }