MLK-18243-14 spi: fspi: init unused LUT to 0

MLK-18243-14 spi: fspi: init unused LUT to 0

If not initialize unused LUT to 0, the value is random which might
cause qspi command failure.

On i.MX8QM/QXP, it works ok because ROM inittialize them to 0, but on
i.MX8MM, ROM not initialize them, so let's do it here.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
(cherry picked from commit c4bd49c7b249073415f052fb28cd5a4ad374a318)
(cherry picked from commit bce752cfe43b3378dbd84b64d46f1d1818e0300f)