rockchip: add option to change method of loading u-boot

If we would like to boot from SD card, we have to implement mmc driver
in SPL stage, and get a slightly large SPL binary. Rockchip SoC's
bootrom code has the ability to load spl and u-boot, then boot.

If CONFIG_ROCKCHIP_SPL_BACK_TO_BROM is enabled, the spl will return to
bootrom in board_init_f(), then bootrom loads u-boot binary.

Loading sequence after rework:
bootrom ==> spl ==> bootrom ==> u-boot

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
Fixed up spelling of U-Boot, boorom, opinion->option, Rochchip:
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-rockchip/rk3288-board-spl.c b/arch/arm/mach-rockchip/rk3288-board-spl.c
index 123f58b..08152aa 100644
--- a/arch/arm/mach-rockchip/rk3288-board-spl.c
+++ b/arch/arm/mach-rockchip/rk3288-board-spl.c
@@ -149,7 +149,7 @@
 	return 0;
 }
 #endif
-
+extern void back_to_bootrom(void);
 void board_init_f(ulong dummy)
 {
 	struct udevice *pinctrl;
@@ -204,6 +204,9 @@
 		debug("DRAM init failed: %d\n", ret);
 		return;
 	}
+#ifdef CONFIG_ROCKCHIP_SPL_BACK_TO_BROM
+	back_to_bootrom();
+#endif
 }
 
 static int setup_led(void)