memory: mtk-smi: add larbid handle routine

In the commit 3c8f4ad8 ("memory/mediatek: add support for mt2701"),
the larb->larbid was added but not initialized.
Mediatek's gen1 smi need this hardware larbid information to get the
register offset which controls whether enable iommu for this larb.
This patch add the initialize routine for larbid.

Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>