arm64: dts: rockchip: Add vcc-supply to SPI flash on rk3566-pinetab2

arm64: dts: rockchip: Add vcc-supply to SPI flash on rk3566-pinetab2

As described on page 37 of PineTab2 Schematic-20230417, the SPI Flash's
VCC connector is connected to VCCIO_FLASH and according to page 6 of
that same schematic, that belongs to the VCC_1V8 power source.

This fixes the following warning:

  spi-nor spi4.0: supply vcc not found, using dummy regulator

Signed-off-by: Diederik de Haas <didi.debian@cknow.org>
Link: https://lore.kernel.org/r/20250425092601.56549-4-didi.debian@cknow.org
Signed-off-by: Heiko Stuebner <heiko@sntech.de>