ARM: clocksource: Add support for MOXA ART SoCs

This patch adds an clocksource driver for the main timer(s)
found on MOXA ART SoCs.

The MOXA ART SoC provides three separate timers with individual
count/load/match registers, two are used here:

TIMER1: clockevents, used to support oneshot and periodic events
TIMER2: set up as a free running counter, used as clocksource

Timers are preconfigured by bootloader to count down and interrupt
on match or zero. Count increments every APB clock cycle and is
automatically reloaded when it reaches zero.

Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>