soc/sifive/fu540: Add missing '#include <commonlib/bsd/helpers.h>'

This is used for 'KHz' (line #19)

Change-Id: I4d610607b50d2fac1150deaaf94f3cb331540fbc
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41151
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Hug <philipp@hug.cx>

src/soc/sifive/fu540/uart.c

@@ -4,6 +4,7 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <console/uart.h>
+#include <commonlib/bsd/helpers.h>
 #include <soc/addressmap.h>
 #include <soc/clock.h>