dev, arm: fix error class-memaccess with GCC >= 8.1

dev, arm: fix error class-memaccess with GCC >= 8.1

From GCC 8.1 on GCC issues a warning when using memset et al on structs and
classes. With the way gem5 builds, this actually prevents successful
builds.

Instead of using a pointer with SCSIReply as type, we cast to a void
pointer to avoid the message. On the way we wrap the memset call into a
method of SCSIReply called reset for better code readability.

Signed-off-by: Maurice Becker <madnaurice@googlemail.com>
Change-Id: I3ed3fd9714be5d253aba01ca00b1863e1ae5cb68
Reviewed-on: https://gem5-review.googlesource.com/12685
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>