[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>