rockchip: Enable building a SPI ROM image on bob

Add a simple binman config and enable CONFIG_HAS_ROM so that U-Boot
produces a ROM for bob.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/dts/rk3399-gru-u-boot.dtsi b/arch/arm/dts/rk3399-gru-u-boot.dtsi
index 7bddc3a..390ac2b 100644
--- a/arch/arm/dts/rk3399-gru-u-boot.dtsi
+++ b/arch/arm/dts/rk3399-gru-u-boot.dtsi
@@ -4,3 +4,7 @@
  */
 
 #include "rk3399-u-boot.dtsi"
+
+&spi_flash {
+	u-boot,dm-pre-reloc;
+};