arch-riscv: Add missing license paragraphs

Some of the files in earlier patches rearranging instruction definitions
were missing copyright and license information. This patch adds them.

Change-Id: I2ac4910a415de6032fc0b7d4422904c682e0ad87
Reviewed-on: https://gem5-review.googlesource.com/6183
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

arch-riscv: Move static_inst into a directory

This patch creates an "insts" directory in src/arch/riscv to store
static portions of instruction definitions that aren't part of the code
generated by the ISA description. It serves as a starting point for
future patches to simplify the ISA description.

Change-Id: I6700522143f6fa6c9b18a30e1fbdc8f80cdc7afa
Reviewed-on: https://gem5-review.googlesource.com/6021
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>