sim: Implement load_addr_mask auto-calculation

sim: Implement load_addr_mask auto-calculation

Recent Linux kernels for AArch64 have changed their start addresses
but we still want to relocate the kernel to 0x80080000 which
required hacking the load_addr_mask in Realview.py to be 0x7ffffff
from 0xfffffff to mask off the proper number of MSBs to load the
kernel in the desired location.  To avoid having to make this change
in the future again, we auto-calculate  the load_addr_mask if it is
specified as 0x0 in the System sim-object to find the most restrictive
address mask instead of having the configuration specify it.  If the
configuration does specify the address mask, we use it instead of
auto-calculating.

Change-Id: I18aabb5d09945c6e3e3819c9c8036ea24b6c35cf
Signed-off-by: Geoffrey Blake <Geoffrey.Blake@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2323
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>