Add rivest cipher 4 (rc4) implementation

Add an implementation of RC4. This will be used by Rockchip booting but may
be useful in other situations.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/Makefile b/lib/Makefile
index 1139f9b..fd106b9 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -37,6 +37,7 @@
 obj-y += net_utils.o
 obj-$(CONFIG_PHYSMEM) += physmem.o
 obj-y += qsort.o
+obj-y += rc4.o
 obj-$(CONFIG_SHA1) += sha1.o
 obj-$(CONFIG_SUPPORT_EMMC_RPMB) += sha256.o
 obj-$(CONFIG_SHA256) += sha256.o