rockchip: Use an external TPL binary on RK3588

There is no support to initialize DRAM on RK3588 SoCs using U-Boot TPL
and instead an external TPL binary must be used to generate a bootable
u-boot-rockchip.bin image.

Enable ROCKCHIP_EXTERNAL_TPL by default for RK3588, add build steps for
RK3588 to documentation and clean up CONFIG_BINMAN_FDT options.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Jagan Teki <jagan@edgeble.ai>
diff --git a/arch/arm/mach-rockchip/Kconfig b/arch/arm/mach-rockchip/Kconfig
index 0390431..e5ac58a 100644
--- a/arch/arm/mach-rockchip/Kconfig
+++ b/arch/arm/mach-rockchip/Kconfig
@@ -428,7 +428,7 @@
 
 config ROCKCHIP_EXTERNAL_TPL
 	bool "Use external TPL binary"
-	default y if ROCKCHIP_RK3568
+	default y if ROCKCHIP_RK3568 || ROCKCHIP_RK3588
 	help
 	  Some Rockchip SoCs require an external TPL to initialize DRAM.
 	  Enable this option and build with ROCKCHIP_TPL=/path/to/ddr.bin to