target/mips/mips-defs: Use ISA_MIPS32R6 definition to check Release 6

target/mips/mips-defs: Use ISA_MIPS32R6 definition to check Release 6

Use the single ISA_MIPS32R6 definition to check if the Release 6
ISA is supported, whether the CPU support 32/64-bit.

For now we keep '32' in the definition name, we will rename it
as ISA_MIPS_R6 in few commits.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210104221154.3127610-11-f4bug@amsat.org>