mem, arm: Replace the pointer type in PortProxy with void *.

The void * type is for pointers which point to an unknown type. We
should use that when handling anonymous buffers in the PortProxy
functions, instead of uint8_t * which points to bytes.

Importantly, C/C++ doesn't require you to do any casting to turn an
arbitrary pointer type into a void *. This will get rid of lots of
tedious, verbose casting throughout the code base.

Change-Id: Id1adecc283c866d8e24524efd64f37b079088bd9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18571
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

mem, arm: Move some helper methods into the base PortProxy class.

These were originally in the SETranslatingPortProxy class, but they're
not specific to SE mode in any way and are an unnecessary divergence
between the SE and FS mode translating port proxies.

Change-Id: I8cb77531cc287bd15b2386410ffa7b43cdfa67d0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18570
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

arm, mem: Move the SecurePortProxy subclass into it's own file.

The idea of a "secure" memory area/access is specific to ARM and
shouldn't be in the common mem directory, although it's built in to the
generic memory protocol at this point.

Regardless, it should minimially be in its own file like the virtual
and physical port proxy classes are.

Change-Id: I140d4566ee2deded784adb04bcf6f11755a85c0c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18569
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>