ARM: dts: rockchip: fix vref-supply for &saradc node rk3288 firefly reload

ARM: dts: rockchip: fix vref-supply for &saradc node rk3288 firefly reload

A test with the command below gives this error:

arch/arm/boot/dts/rk3288-firefly-reload.dt.yaml: saradc@ff100000:
'vref-supply' is a required property

PMIC Channel OUT11 with powername 'vcc_18'
(connected through R155 bridge with 'vccio_wl')
is used for the recovery key and ADC_AVDD_1V8.

Fix error by adding 'vcc_18' as vref for the saradc.

make ARCH=arm dtbs_check
DT_SCHEMA_FILES=Documentation/devicetree/bindings/iio/adc/
rockchip-saradc.yaml

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Link: https://lore.kernel.org/r/20200314140755.4877-1-jbx6244@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>