mailbox: Rename free() to rfree()

This function name conflicts with our desire to #define free() to
something else on sandbox. Since it deals with resources, rename it to
rfree().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mailbox/k3-sec-proxy.c b/drivers/mailbox/k3-sec-proxy.c
index b07b56c..1194c6f 100644
--- a/drivers/mailbox/k3-sec-proxy.c
+++ b/drivers/mailbox/k3-sec-proxy.c
@@ -291,7 +291,7 @@
 struct mbox_ops k3_sec_proxy_mbox_ops = {
 	.of_xlate = k3_sec_proxy_of_xlate,
 	.request = k3_sec_proxy_request,
-	.free = k3_sec_proxy_free,
+	.rfree = k3_sec_proxy_free,
 	.send = k3_sec_proxy_send,
 	.recv = k3_sec_proxy_recv,
 };