target/mips: Move cpu_mips_get_random() with CP0 helpers

target/mips: Move cpu_mips_get_random() with CP0 helpers

The get_random() helper uses the CP0_Wired register, which is
unrelated to the CP0_Count register used as timer.
Commit e16fe40c872 ("Move the MIPS CPU timer in a separate file")
incorrectly moved this get_random() helper with timer specific
code. Move it back to generic CP0 helpers.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-Id: <20201012095804.3335117-6-f4bug@amsat.org>