pinctrl: pinctrl-at91: Demote non-kerneldoc header and complete another

pinctrl: pinctrl-at91: Demote non-kerneldoc header and complete another

The documentation header for 'struct at91_pinctrl_mux_ops' was missing
entries for {g,s}et_drivestrength and {g,s}et_slewrate.

Fixes the following W=1 kernel build warning(s):

 drivers/pinctrl/pinctrl-at91.c:77: warning: cannot understand function prototype: 'enum drive_strength_bit '
 drivers/pinctrl/pinctrl-at91.c:187: warning: Function parameter or member 'get_drivestrength' not described in 'at91_pinctrl_mux_ops'
 drivers/pinctrl/pinctrl-at91.c:187: warning: Function parameter or member 'set_drivestrength' not described in 'at91_pinctrl_mux_ops'
 drivers/pinctrl/pinctrl-at91.c:187: warning: Function parameter or member 'get_slewrate' not described in 'at91_pinctrl_mux_ops'
 drivers/pinctrl/pinctrl-at91.c:187: warning: Function parameter or member 'set_slewrate' not described in 'at91_pinctrl_mux_ops'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Link: https://lore.kernel.org/r/20200713144930.1034632-18-lee.jones@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>