rockchip: Move bootrom-related declarations to a header

So far spl-boards have declared the back_to_brom() function as simple
extern in the files themself. That doesn't scale well if every boards
defines this on its own.
Therefore move the declarations to a bootrom header.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/rk3288-board-spl.c b/arch/arm/mach-rockchip/rk3288-board-spl.c
index 3713c13..74f3379 100644
--- a/arch/arm/mach-rockchip/rk3288-board-spl.c
+++ b/arch/arm/mach-rockchip/rk3288-board-spl.c
@@ -14,6 +14,7 @@
 #include <spl.h>
 #include <asm/gpio.h>
 #include <asm/io.h>
+#include <asm/arch/bootrom.h>
 #include <asm/arch/clock.h>
 #include <asm/arch/hardware.h>
 #include <asm/arch/periph.h>
@@ -155,7 +156,7 @@
 	return 0;
 }
 #endif
-extern void back_to_bootrom(void);
+
 void board_init_f(ulong dummy)
 {
 	struct udevice *pinctrl;