arm64: defconfig: Enable REGULATOR_MP8859

RK3399 boards like ROC-RK3399-PC is using MP8859 DC/DC converter
for 12V supply.

roc-rk3399-pc initially used 12V fixed regulator for this supply,
but the below commit has switched to use MP8859.

commit <1fc61ed0> "arm64: dts: rockchip:
Enable mp8859 regulator on rk3399-roc-pc"

So, enable by default on the defconfig.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Suniel Mahesh <sunil@amarulasolutions.com>
Link: https://lore.kernel.org/r/20200731160324.142097-1-jagan@amarulasolutions.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>