pinctrl: bcm: Constify static pinmux_ops

These are only assigned, either directly or via the bcm63xx_pinctrl_soc
struct, to the pmxops field in the pinctrl_desc struct and never
modified, so make them const to allow the compiler to put them in
read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210605185908.39982-3-rikard.falkeborn@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>