serial: omap: Request pins using pinctrl framework

Request pins using pinctrl framework. Only show a warning
on error as some boards set the pins in the bootloader
even if CONFIG_PINCTRL is enabled.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>