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>

board/armltd/vexpress/vexpress_common.c

@@ -119,11 +119,6 @@ void dram_init_banksize(void)
 			get_ram_size((long *)PHYS_SDRAM_2, PHYS_SDRAM_2_SIZE);
 }
 
-int timer_init(void)
-{
-	return 0;
-}
-
 /*
  * Start timer:
  *    Setup a 32 bit timer, running at 1KHz