soc/sifive/fu540: Initialize SDRAM

Based on SiFive bootloader code

Change-Id: I71043ce9e458e25e64da28d53cd36b02d2e22acc
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/28604
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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

@@ -40,5 +40,7 @@ void main(void)
 		uart_init(CONFIG_UART_FOR_CONSOLE);
 	#endif
 
+	sdram_init();
+
 	run_ramstage();
 }