target/mips: Extract FPU specific definitions to translate.h

Extract FPU specific definitions that can be used by
ISA / ASE / extensions to translate.h header.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201214183739.500368-16-f4bug@amsat.org>