serial: sh-sci: Remove unused GPIO request code

The driver requests at initialization time GPIOs passed through platform
data. No platform makes use of this feature, remove it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>