mainstone: Fix duplicate array values for key 'space'

mainstone: Fix duplicate array values for key 'space'

cgcc reported a duplicate initialisation. Mainstone includes a matrix
keyboard where two different positions map to 'space'.

QEMU uses the reversed mapping and does not map 'space' to two different
matrix positions.

Some other keys are either missing or might be mapped wrongly (cf. Linux
kernel code). Don't fix these until someone can test them with real
hardware, but add TODO comments.

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>