gpio: Handle stmpe-gpio platform data with no gpio base

This patch removes the requirement that gpio base be supplied in
platform data.

Signed-off-by: Virupax Sadashivpetimath <virupax.sadashivpetimath@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>