arc: clarify in comment that ARC must have timer0

The timer implementation for ARC currently requires timer0 to be present.
I've added a comment that this is an assumption and to encourage developers
to build the ARC CPU with Timer0, when it is to be used with Zephyr.
There is also an optional provision for a Timer1. In future, this
code could be conditional to use either timer.

Change-Id: I4eb3aec59ba4e85f8b70d5531b21bdaab00b93bb
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>