iommu/ipmmu-vmsa: IMUCTRn.TTSEL needs a special usage on R-Car Gen3

The TTSEL bit of IMUCTRn register of R-Car Gen3 needs to be set
unused MMU context number even if uTLBs are disabled
(The MMUEN bit of IMUCTRn register = 0).
Since initial values of IMUCTRn.TTSEL on all IPMMU-domains are 0,
this patch adds a new feature "reserved_context" to reserve IPMMU
context number 0 as the unused MMU context.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>