pinctrl: mvebu: prevent walking off the end of group array

While investigating (ab)use of krealloc, David found this bug.  It's
unlikely to occur, but now we detect the condition and error out
appropriately.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>