pinctrl: imx: reset group index on probe

Group index is incremented on every new group parsed. Since the
field is part of struct imx_pinctrl_soc_info, which is typically
a global variable passed by the individual pinctrl-imx.c based
driver, it does not get cleared automatically when re-probing the
driver. This lead imx_pinctrl_parse_functions passing a group
pointer which is outside of the allocated group space on second
probe and onwards. Typically this ended up in a NULL pointer
dereference when accessing the name field like this:
Unable to handle kernel NULL pointer dereference at virtual address 00000000
...
PC is at strcmp+0x18/0x44
LR is at imx_dt_node_to_map+0xc4/0x290

Avoid this by setting group_index to 0 on probe.

This has been observed when using DEBUG_TEST_DRIVER_REMOVE.
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/freescale/pinctrl-imx.c

@@ -687,6 +687,7 @@ static int imx_pinctrl_probe_dt(struct platform_device *pdev,
 	if (!info->functions)
 		return -ENOMEM;
 
+	info->group_index = 0;
 	if (flat_funcs) {
 		info->ngroups = of_get_child_count(np);
 	} else {