board_f: powerpc: Move prt_83xx_rsr() to private code

This function is called just before checkcpu() on MPX83xx. Move it to the
code for that arch.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/common.h

@@ -712,8 +712,6 @@ ulong cpu_init_f(void);
 int	cpu_init_r    (void);
 #if defined(CONFIG_MPC8260)
 int	prt_8260_rsr  (void);
-#elif defined(CONFIG_MPC83xx)
-int	prt_83xx_rsr  (void);
 #endif
 
 /* $(CPU)/interrupts.c */