ARM: davinci: pass clock as parameter to davinci_timer_init()

This changes davinci_timer_init() so that we pass the clock as a
parameter instead of using clk_get(). This is done in preparation
for converting to the common clock framework.

It removes the requirement that we have to have a clock with con_id
of "timer0", which will be good for DT bindings since clock-names =
"timer0" doesn't really make sense.

Also, drop use of extern in header file since we are touching the
definition.

Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>