target/mips: Extract msa_translate_init() from mips_tcg_init()

The msa_wr_d[] registers are only initialized/used by MSA.

They are declared static. We want to move them to the new
'msa_translate.c' unit in few commits, without having to
declare them global (with extern).

Extract first the logic initialization of the MSA registers
from the generic initialization. We will later move this
function along with the MSA registers to the new C unit.

Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20201208003702.4088927-8-f4bug@amsat.org>