ARM: dts: sun5i: reference-design-tablet: Set lradc vref to avcc

Set lradc vref to the actual avcc ldo, rather then to the fixed 3v0
regulator from common-regulators.dtsi.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>