arm64: dts: rockchip: rk3568: Add sfc a spi4 alias

Kernel's SFC is used for a private flash drivers not like
the standard one, but u-boot SFC is register as standard
spi, and the alias UCLASS_SPI id is required for part of
spi initialization progress. So kernel's alias is also
required when u-boot enable using kernel dtb case.

Change-Id: Ic763eefb35ef5a93f6121055a5e04a2d72636430
Signed-off-by: Jon Lin <jon.lin@rock-chips.com>

arch/arm64/boot/dts/rockchip/rk3568.dtsi

@@ -59,6 +59,7 @@
 		spi1 = &spi1;
 		spi2 = &spi2;
 		spi3 = &spi3;
+		spi4 = &sfc; // for U-Boot
 	};
 
 	cpus {