ARM: mxs: use apbx bus clock to drive the timers on timrotv2

timer resolution of ~32us is pretty low.
v2 has 32bits resolution, so we have quite some headroom, and
can use the 24MHz clock.
v1 has only 16bits, so we only increase v2.

So we just exchange the timrot clock in imx28.
On imx23 we have timrotv1 and everything stays the same.

Signed-off-by: Torben Hohn <torbenh@linutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>