counter: Fix interface

With the current interface if multiple counters/timers are present in
the system, it is not possible to select from which one to read the
current value.

This patch fixes the behavior.

Change-Id: Id1ae1f2330e98d078f755c0b81c3b176e90b8389
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>