fastmodel: Plumb the ITB and DTB through the IRIS thread context.

fastmodel: Plumb the ITB and DTB through the IRIS thread context.

These might be necessary to, for instance, translate virtual addresses.
A custom TLB which uses the IRIS API will be written which can be
substituted in for the normal ARM TLB.

Change-Id: Ic44822db6692ca3a4ca13875b2260b08547a24da
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22116
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>