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 <[e-mail]>
Cc: Ludovic Desroches <[e-mail]>
Cc: Nicolas Ferre <[e-mail]>
Cc: Alexandre Belloni <[e-mail]>
Cc: Jean-Christophe PLAGNIOL-VILLARD <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

drivers/pinctrl/pinctrl-at91.c

···
 #define DEBOUNCE_VAL_SHIFT	17
 #define DEBOUNCE_VAL	(0x3fff << DEBOUNCE_VAL_SHIFT)
 
-/**
+/*
  * These defines will translated the dt binding settings to our internal
  * settings. They are not necessarily the same value as the register setting.
  * The actual drive strength current of low, medium and high must be looked up
···
  * @set_pulldown: enable/disable pulldown
  * @get_schmitt_trig: get schmitt trigger status
  * @disable_schmitt_trig: disable schmitt trigger
+ * @get_drivestrength: get driver strength
+ * @set_drivestrength: set driver strength
+ * @get_slewrate: get slew rate
+ * @set_slewrate: set slew rate
  * @irq_type: return irq type
  */
 struct at91_pinctrl_mux_ops {