drivers: timer: Introduce ARC timer driver

This commit introduces timer driver for ARC.

ARC timers are configured via ARC AUX registers so we use special
functions to access timer control registers.

This driver allows utilization of either timer0 or timer1
depending on which one is available in real hardware. Essentially
only existing timers should be mentioned in board's Device Tree
description.

Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>