mailbox: explicitly include <linux/bits.h>

Don't rely on those including the header file to already include the
needed <linux/bits.h>. Include it in the header file.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: ambarus

drivers/mailbox/mailbox.h

@@ -3,6 +3,8 @@
 #ifndef __MAILBOX_H
 #define __MAILBOX_H
 
+#include <linux/bits.h>
+
 #define TXDONE_BY_IRQ	BIT(0) /* controller has remote RTR irq */
 #define TXDONE_BY_POLL	BIT(1) /* controller can read status of last TX */
 #define TXDONE_BY_ACK	BIT(2) /* S/W ACK received by Client ticks the TX */