linux: prepare defconfigs for axp20x thermal support

Drop CONFIG_HWMON=m and CONFIG_HWMON=n and use the default value. This
mirrors:

	https://github.com/linux-sunxi/linux-sunxi/commit/5a467f646bd75dc2ef3f4e4bafbef456472dca9e

and prepares for:

	https://github.com/linux-sunxi/linux-sunxi/commit/fcec507519157765c689ab3473a9e72d8b6df453

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>