arm: omap2: timer: remove __omap_gptimer_init()

__omap_sync32k_timer_init(), now takes the clock
source as a parameter. This means we no longer need
__omap_gptimer_init().

Note that __omap_sync32k_timer_init() will be
renamed in a follow-up patch as it's not longer 32k
source specific.

Signed-off-by: Felipe Balbi <balbi@ti.com>