We only use the three low-order mailbox bits.  Leave the upper bits alone
for possible use by drivers and other software.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2090/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>