Add a mailbox interface.

Mailboxes are an IPC mechanism found on several ARM architectures, namely
OMAP4, BCM2385, Sitara, etc.

Reviewed by:	gonzo

sys/dev/mbox/mbox_if.m

@@ -0,0 +1,46 @@
+#-
+# Copyright (c) 2013 Rui Paulo
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+#
+# $FreeBSD$
+#
+# Mailboxes are used as an IPC mechanism that allows subsystems to 
+# communicate with each other.
+# A system can have multiple mailboxes (e.g. OMAP4) but there's usually a
+# system wide mailbox.
+#
+
+INTERFACE mbox;
+
+METHOD int read {
+	device_t	dev;
+	int		channel;
+	uint32_t	*data;
+};
+
+METHOD int write {
+	device_t	dev;
+	int		channel;
+	uint32_t	data;
+};