gpio: mvebu: Don't free chip label memory

The gpio_chip.label field is a const char * and assigned the value of a
call to dev_name(). Memory obtained from dev_name() should not be freed
by drivers.

Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>