Move timer register helpers from arch.h to arch_helpers.h

Some of the affected macros can only be used from C code. In general, we
use arch_helpers.h for any C helpers to access registers. For
consistency, the other macros have been moved as well.

Change-Id: If27ee82b067d920d7b338c0a1b6e61a6ec078f4f
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>