mfd: stmpe: DT: Add stmpe-i2c dt alias to get id device

This patch augments the STMP driver to read the device id
from the stmpe-i2c dt alias if present.

Cc: devicetree-discuss@lists.ozlabs.org
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>