target/sh4: movua.l is an SH4-A only instruction

At the same time change the comment describing the instruction the same
way than other instruction, so that the code is easier to read and search.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>