serial/imx: improve error diagnosics for clock and pinctrl failures

These error paths are used more often now after deep changes to the
clock code and pinctrl is still new for imx. So help debugging and give
clues in the boot log.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>