ARM: Fix broken USB support in sunxi_defconfig

Commit 69fb4dca ("power: Add an axp20x-usb-power driver") introduced a new
driver for the USB power supply used on various Allwinner based SBCs. However,
the driver was not added to sunxi_defconfig which breaks USB support for some
boards (e.g. LeMaker BananaPi) as the kernel will now turn off the USB power
supply during boot by default if the driver isn't present. (This was not the
case in linux 4.3 or lower where the USB power was always left on.)

Hence, add the driver to sunxi_defconfig in order to keep USB support working
on those boards that require it.

Signed-off-by: Timo Sigurdsson <public_timo.s@silentcreek.de>
Reported-by: David Tulloh <david@tulloh.id.au>
Tested-by: David Tulloh <david@tulloh.id.au>
Tested-by: Timo Sigurdsson <public_timo.s@silentcreek.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>