ARM: meson6: clocksource: Add Meson6 timer support

Meson6 SoCs are equipped with 5 32-bit timers, called TIMER_A, TIMER_B,
TIMER_C, TIMER_D and TIMER_E.

The driver is providing clocksource support for the 32-bit counter using
TIMER_E. Clockevents are also supported using TIMER_A.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Carlo Caione <carlo@caione.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>