sparc: Pull most of the Nop format out of the ISA description.

The Nop format mostly just made instructions that inherited from the
Nop base class but with different mnemonics, so there doesn't need
to be very much dynamic content.

Change-Id: I1cf5e25ca8372f9b71f56d49756879c7545c9f6c
Reviewed-on: https://gem5-review.googlesource.com/5462
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>