arm: omap2: timer: always define omap4_local_timer_init

omap4_local_timer_init() can be used by other
platforms as is. At least AM437x wants to use
it. Instead of making omap4-only and providing
a stub for builds without OMAP4, we can just
make sure that function is always available
for all SoCs that need it.

Reported-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>