video: fbdev: pxafb: match_string() conversion fixup

"unknown" lcd_types[] entry is needed for proper operation of
the driver, add it back.

Fixes: 6d09dfe7 ("video: fbdev: pxafb: Convert to use match_string() helper")
Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>