PA-RISC already handled the dummy console screen size selection in
Kconfig, so generalize this to other platforms.

ARM keeps on using screen_info, which is filled in by
platform-specific code, or from ATAGS.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>