serial: lpc32xx_hs: Drop surplus include

The driver includes <linux/gpio.h> but does not use any symbols
from the file so drop this include.

Cc: Roland Stigge <stigge@antcom.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200415184300.269889-1-linus.walleij@linaro.org


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