serial/imx: add device tree probe support

It adds device tree probe support for imx tty/serial driver.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Jason Liu <jason.hui@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Grant Likely <grant.likely@secretlab.ca>