arm64: add defconfig options for Allwinner SoCs

With the Allwinner platform now supported, enable it in the defconfig
and add some options to give some decent out-of-the-box experience on
those SoCs.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>