mailbox: zynq: make modular

This driver has a MODULE_LICENSE but is not tristate so cannot be
built as a module, unlike all its peers: make it modular to match.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Suggested-by: Michal Simek <michal.simek@xilinx.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: linux-modules@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Hitomi Hasegawa <hasegawa-hitomi@fujitsu.com>
Cc: Jassi Brar <jassisinghbrar@gmail.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>