sim-se: const for loader's loadSection param

sim-se: const for loader's loadSection param

The port proxy can be declared as a reference to a const proxy
rather than just a reference to a proxy.

Change-Id: I4640b0c5f33e2334c1e7630131f78607ced40a34
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/12301
Maintainer: Brandon Potter <Brandon.Potter@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>