serial: omap: Convert to use GPIO descriptors

This converts the OMAP serial driver to use a GPIO descriptor
for the optional RTS signal.

Cc: Shubhrajyoti D <shubhrajyoti@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200415183927.269445-1-linus.walleij@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>