target/mips: Move translate.h to tcg/ sub directory

We moved various TCG source files in commit a2b0a27d
("target/mips: Move TCG source files under tcg/ sub directory")
but forgot to move the header declaring their prototypes.
Do it now, since all it declares is TCG specific.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210617174323.2900831-5-f4bug@amsat.org>