mfd: stmpe: Support gpio over irq under device tree

The stmpe_platform_data has a irq_over_gpio field, which allows the
system to read STMPE events whenever an IRQ occurs on a GPIO pin.
This patch adds the ability to configure this field and to use a GPIO
as an IRQ source for boards configuring the STMPE in device tree.

Signed-off-by: Sean Cross <xobs@kosagi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>