pinctrl: at91-pio4: account for const type of of_device_id.data

This driver creates a const structure that it stores in the data field
of an of_device_id array.

Adding const to the declaration of the location that receives the
const value from the data field ensures that the compiler will
continue to check that the value is not modified.  Furthermore, the
const-discarding cast on the extraction from the data field is no
longer needed.

Done using Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>