watchdog: imx: Support set timeout by wdt command

After "4b969dea watchdog: imx: Add DM support", the imx watchdog
can be started by wdt command. But the imx watchdog driver only
support start with the default timeout.

This commit adds the support for setting the timeout which pass from
the wdt command into the imx watchdog. If the timeout out of the
valid range(0.5~128s), start the watchdog with a timeout within the
valid range and the timeout is the one which closest to the passed
timeout.

Signed-off-by: Yuezhang.Mo <yuezhang.mo@sony.com>
Reviewed-by: Andy.Wu <Andy.Wu@sony.com>
Reviewed-by: stefano Babic <sbabic@denx.de>