lib: time: add weak timer_init() function

If timer_init() is made a weak stub function, then it allows us to
remove several empty timer_init functions for those boards that
already have a timer initialized when u-boot starts. Architectures
that use the timer framework may also remove the need for timer.c.

Signed-off-by: Darwin Rambo <drambo@broadcom.com>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>