iommu/ipmmu-vmsa: Move IMTTBCR_SL0_TWOBIT_* to restore sort order

Move the recently added IMTTBCR_SL0_TWOBIT_* definitions up, to make
sure all IMTTBCR register bit definitions are sorted by decreasing bit
index.  Add comments to make it clear that they exist on R-Car Gen3
only.

Fixes: c295f504 ("iommu/ipmmu-vmsa: Allow two bit SL0")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Joerg Roedel <jroedel@suse.de>