clocksource/drivers/timer-milbeaut: Introduce timer for Milbeaut SoCs

Add timer driver for Milbeaut SoCs series.

The timer has two 32-bit width down counters, one of which is configured
as a clockevent device and the other is configured as a clock source.

Signed-off-by: Sugaya Taichi <sugaya.taichi@socionext.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>