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>

arch/arm/include/asm/arch-rockchip/bootrom.h

@@ -0,0 +1,22 @@
+/*
+ * (C) Copyright 2017 Heiko Stuebner <heiko@sntech.de>
+ *
+ * SPDX-License-Identifier:	GPL-2.0
+ */
+
+#ifndef _ASM_ARCH_BOOTROM_H
+#define _ASM_ARCH_BOOTROM_H
+
+/*
+ * Saved Stack pointer address.
+ * Access might be needed in some special cases.
+ */
+extern u32 SAVE_SP_ADDR;
+
+/*
+ * Hand control back to the bootrom to load another
+ * boot stage.
+ */
+extern void back_to_bootrom(void);
+
+#endif