sparc: Update cpu_init.c to use generic timer infrastructure

Introduce the CONFIG_SYS_TIMER_* macros in include/asm/config.h to make use
of the generic timer infrastructure in lib/time.c.

Created a timer_init() function to initialize the timer hardware and update
the #ifdef in board_init_f to allow this function to be called during the
start-up sequence.

Signed-off-by: Francois Retief <fgretief@spaceteq.co.za>