pinctrl: at91: Constify struct at91_pinctrl_mux_ops

The at91_pinctrl_mux_ops struct is never modified by the driver. Make it
const wherever it is possible to allow the compiler to put the static
variables in read-only memory. Note that sam9x60_ops was already const,
but the const was cast away when the return value of of_match_device() was
cast to a pointer to a non-const struct at91_pinctrl_mux_ops.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20210512180140.33293-1-rikard.falkeborn@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>