[MIPS] Add an appropriate header into display.c

The following errors were caught by sparse:

>>>>>>>>>>>

arch/mips/mips-boards/generic/display.c:30:6: warning: symbol
'mips_display_message' was not declared. Should it be static?

arch/mips/mips-boards/generic/display.c:58:6: warning: symbol
'mips_scroll_message' was not declared. Should it be static?

>>>>>>>>>>>

This patch includes the asm/mips-boards/prom.h header file into
arch/mips/mips-boards/generic/display.c. This adds the needed
function declarations, and the errors are gone.

Compile-tested using defconfigs for Malta, Atlas and SEAD boards.
Runtime test was successfully performed by booting a Malta 4Kc
board up to the shell prompt.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mips-boards/generic/display.c

@@ -22,6 +22,7 @@
 #include <linux/timer.h>
 #include <asm/io.h>
 #include <asm/mips-boards/generic.h>
+#include <asm/mips-boards/prom.h>
 
 extern const char display_string[];
 static unsigned int display_count;