gpio: Add bitmask to block requests to unavailable stmpe GPIOs

GPIOs on these controller are multi-functional. If you decided to use
some of them e.g. as input channels for the ADC, you surely don't want
those pins to be reassigned as simple GPIOs (which may be triggered even
from userspace via 'export'). Same for the touchscreen controller pins.
Since knowledge about the hardware is needed to decide which GPIOs to
reserve, let this bitmask be inside platform_data and provide some
defines to assist potential users.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Rabin Vincent <rabin.vincent@stericsson.com>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>