target/mips: Explicit Release 6 MMU types

target/mips: Explicit Release 6 MMU types

As of Release 6, MMU type 4 is assigned to "Dual Variable-Page-Size
and Fixed-Page-Size TLBs" and type 2 to "Block Address Translation.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201201132817.2863301-4-f4bug@amsat.org>