pxa27x: Reduce size of keyboard matrix mapping

The row and column values use only a very limited range (-1 ... 7),
so a byte value is sufficient.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
