fbcon: Increase maximum font width x height to 64 x 128

By using bitmaps we actually support whatever size we would want, but
the console currently limits fonts to 64x128 (which gives 60x16 text on
4k screens), so we don't need more for now, and we can easily increase
later.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Helge Deller <deller@gmx.de>