arm: Add a prototype for save_boot_params_ret()

It is convenient for some boards to implement save_boot_params() in C rather
than assembler. Provide a way to return in this case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>

arch/arm/include/asm/system.h

@@ -159,6 +159,22 @@ void flush_l3_cache(void);
  * void save_boot_params(u32 r0, u32 r1, u32 r2, u32 r3);
  */
 
+/**
+ * save_boot_params_ret() - Return from save_boot_params()
+ *
+ * If you provide save_boot_params(), then you should jump back to this
+ * function when done. Try to preserve all registers.
+ *
+ * If your implementation of save_boot_params() is in C then it is acceptable
+ * to simply call save_boot_params_ret() at the end of your function. Since
+ * there is no link register set up, you cannot just exit the function. U-Boot
+ * will return to the (initialised) value of lr, and likely crash/hang.
+ *
+ * If your implementation of save_boot_params() is in assembler then you
+ * should use 'b' or 'bx' to return to save_boot_params_ret.
+ */
+void save_boot_params_ret(void);
+
 #define isb() __asm__ __volatile__ ("" : : : "memory")
 
 #define nop() __asm__ __volatile__("mov\tr0,r0\t@ nop\n\t");