ARM i.MX timer: request correct clock

We used to pass the timer clock directly to mxc_timer_init. We
should instead request the correct clock. This is an intermediate
step: For now we request the clock in the timer code when NULL
is passed as clock.
Also, the gpt on some i.MX have an additional ipg clock which can
be gated. Request and enable this.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>