arm64: dts: allwinner: pinebook: Make simplefb more consistent

Boards generally reference the simplefb nodes from the SoC dtsi by
label, not by full path. simplefb_hdmi is already like this in the
Pinebook DTS. Update simplefb_lcd to match.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>