[ARM] 4092/1: i.MX/MX1 CPU Frequency scaling latency definition

The transition latency has to be defined and reasonably
small to allow on-demand and conservative governors.
The value has been defined according to manual.
The imx_set_target() protected against seen out of range
requests now.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>