arch-arm: Fix ArmSystem::_resetAddr evalutation

arch-arm: Fix ArmSystem::_resetAddr evalutation

With:

https://gem5-review.googlesource.com/c/public/gem5/+/26466

The ArmSystem reset address (_resetAddr) is always forced by the
workload:

 _resetAddr = workload->entry

So there is no possibility to manually specify a reset address.

This was not the case before:
The resetAddr was forced only if auto_reset_addr was true or if there
was an associated bootloader to the kernel image. In that case even if
auto_reset_addr was false, the reset address was determined by the
bootloader entry.
This was also not ideal (but it was working)

This patch is cleaning all of this:

If you want to have automatic detection (recommended), you would need to
set auto_reset_addr (now turned to true by default).  This will allow to
keep most fs script untouched.  If you don't want to use automatic
detection, set auto_reset_addr to False and provide your own reset
address.

Change-Id: I5d7a55fd9060b9973c7d5b5542bd199950e1073e
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26723
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>