powerpc: Add Microwatt platform

Microwatt is a FPGA-based implementation of the Power ISA.  It
currently only implements little-endian 64-bit mode, and does
not (yet) support SMP, VMX, VSX or transactional memory.  It has an
optional FPU, and an optional MMU (required for running Linux,
obviously) which implements a configurable radix tree but not
hypervisor mode or nested radix translation.

This adds a new machine type to support FPGA-based SoCs with a
Microwatt core.  CONFIG_MATH_EMULATION can be selected for Microwatt
SOCs which don't have the FPU.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/YMwWbZVREsVug9R0@thinks.paulus.ozlabs.org