refactor(delay-timer): add timer callback functions

In order to avoid separate platform definitions when not using the
default timer functions, it is better to move these functions out of the
header file and into the source files, so that they can be built if
needed.

Move timer functions from delay_timer.h into generic_delay_timer.c. Add
them as callback functions which are then called in delay_timer.c.

Change-Id: I96a1eac8948b1a7b1e481899b67a083db4c9b97d
Signed-off-by: Abhi Singh <abhi.singh@arm.com>
4 files changed