This patch introduces initial support for the new nanoMIPS ISA, the
associated P32 ABI and the MIPS I7200 CPU.

nanoMIPS has a variable-length compressed instruction set that is
completely standalone from the other MIPS ISAs. It is designed to
compress the highest frequency instructions to 16-bits, and use 48-bit
instructions to efficiently encode 32-bit constants into the instruction
stream. There is also a wider range of 32-bit instructions, which merge
carefully chosen high frequency instruction sequences into single
operations creating more flexible addressing modes such as indexed and
scaled indexed addressing, branch compare with immediate and macro style
instructions. The macro like instructions compress prologue and epilogue
sequences, as well as a small number of high frequency instruction pairs
like two move instructions or a move and function call. nanoMIPS also
totally eliminates branch delay slots which follows a precedent set by
microMIPSR6.

Due to the binary incompatibility between previous MIPS architecture
generations and nanoMIPS, and the significantly revamped compiler ABI,
where for the first time, a single Linux kernel would not be expected to
handle both old and new ABIs, we have decided to also take the
opportunity to modernise the Linux user ABI for nanoMIPS, making as much
use of generic interfaces as possible and modernising the true
architecture specific parts.

This is similar to what a whole new kernel architecture would be
expected to adopt, but has been done within the existing MIPS
architecture port to allow reuse of the existing MIPS code, most of
which does not depend on these ABI specifics. Details of the proposed
Linux user ABI changes for nanoMIPS can be found here:

  https://www.linux-mips.org/wiki/P32_Linux_ABI

nanoMIPS pre-built toolchains and source code tarballs are available at:

  http://codescape.mips.com/components/toolchain/nanomips/2018.04-02/

This single patch is being released as the most expedient path to
releasing this work into the wild, but is of course not intended to be
submitted or merged upstream. This work will be prepared & submitted as
series of smaller patches in the coming months.

Co-authored-by: James Hogan <jhogan@kernel.org>
Co-authored-by: Marcin Nowakowski <marcin.nowakowski@mips.com>
Co-authored-by: Matt Redfearn <matt.redfearn@mips.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>