arm64: Enable MP8859 regulator support as module

On the Rockchip Firefly ROC-RK3399-PC board, a MP8859 regulator is used which supplies power to the whole board. Currently the kernel does not boot as all of the other devices further down the power tree deferred probe and wait for the missing driver!

Enable the driver for the regulator as a module such that the board can power up properly.

Signed-off-by: Christopher Obbard chris.obbard@collabora.com