Exynos5420: Initialize USB PHY

... this is needed for libpayload to talk to USB devices.
(forward ported from https://gerrit.chromium.org/gerrit/#/c/55554)

Change-Id: I5a20864689efd0c0149775e6d85b658e0cc6715c
Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Gabe Black <gabeblack@chromium.org>
Reviewed-on: http://review.coreboot.org/3697
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/src/cpu/samsung/exynos5420/Makefile.inc b/src/cpu/samsung/exynos5420/Makefile.inc
index 6f9b91b..8540409 100644
--- a/src/cpu/samsung/exynos5420/Makefile.inc
+++ b/src/cpu/samsung/exynos5420/Makefile.inc
@@ -52,6 +52,7 @@
 ramstage-y += i2c.c
 ramstage-y += dp-reg.c
 ramstage-y += fb.c
+ramstage-y += usb.c
 
 exynos5420_add_bl1: $(obj)/coreboot.pre
 	printf "    DD         Adding Samsung Exynos5420 BL1\n"