ARM: sunxi: configs: Enable the Mailbox driver

The mailbox found on the A31 and later SoCs have recently gained some
support in Linux. Since it's going to be useful for crust, let's enable it
in our defconfig.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20200706132323.147562-2-maxime@cerno.tech