ARM: mxs: Add timer support

There are 2 versions of the timrot on Freescale MXS-based SoCs.
The v1 on MX23 only gets 16 bits counter, while v2 on MX28
extends the counter to 32 bits.

The implementation uses two timers, one for clock_event and
another for clocksource. MX28 uses timrot 0 and 1, while MX23
uses 0 and 2.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>