arm: Add support for tracking TCs in ISA devices

arm: Add support for tracking TCs in ISA devices

ISA devices typically need to keep track of the thread context they
are associated with. Among other things, this is required for
interrupt delivery. Add a BaseISADevice:setThreadContext() method to
wire such models to the right thread context.

Change-Id: Iad354d176c0c4c4e34c6ab8b5acaee0b69da0406
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/12399
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>