hw/timer/arm_mptimer.c: Undo accidental rename of arm_mptimer_init()

hw/timer/arm_mptimer.c: Undo accidental rename of arm_mptimer_init()

In commit b01422622b we did an automated rename of the ptimer_init()
function to ptimer_init_with_bh().  Unfortunately this caught the
unrelated arm_mptimer_init() function.  Undo that accidental
renaming.

Fixes: b01422622b7c7293196fdaf1dbb4f495af44ecf9
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20191017133331.5901-1-peter.maydell@linaro.org