Newer hardware supports extra feature in the whitelist registers. This
patch updates the selftest to test that entries marked as read only
are actually read only.

v2: Removed all use of 'rsvd' for read-only registers to avoid
ambiguous code or error messages.
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
CC: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712070745.35239-3-John.C.Harrison@Intel.com