mem: Add PortProxy read/write helper with explicit endianness

Change-Id: Ia9a11ca68b2892dafd02f2c37324b99b35c77d34
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8146
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>