ARM: OMAP2+: ads7846_init: put gpio_pendown into pdata if it's provided

If platform data is provided by the caller gpio_pendown is put into
unused static ads7846_config structure and effectively has no effect.
Of course caller can set gpio_pendown field in platform data himself
but it seems natural to do this in ads7846_init to remove duplication.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>