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>

drivers/mfd/stmpe.c

@@ -1015,7 +1015,10 @@ void stmpe_of_probe(struct stmpe_platform_data *pdata, struct device_node *np)
 {
 	struct device_node *child;
 
-	pdata->id = -1;
+	pdata->id = of_alias_get_id(np, "stmpe-i2c");
+	if (pdata->id < 0)
+		pdata->id = -1;
+
 	pdata->irq_trigger = IRQF_TRIGGER_NONE;
 
 	of_property_read_u32(np, "st,autosleep-timeout",