drivers/pinctrl/pinctrl-at91.c: convert kfree to devm_kfree

The function at91_dt_node_to_map is ultimately called by the function
pinctrl_get, which is an exported function.  Since it is possible that this
function is not called from within a probe function, for safety, the kfree
is converted to a devm_kfree, to both free the data and remove it from the
device in a failure situation.

Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>