pinctrl: mvebu: pinctrl-armada-37xx: Update documentation block for 'struct armada_37xx_pin_group'

Correct misspellings and provide missing entries.

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

 drivers/pinctrl/mvebu/pinctrl-armada-37xx.c:68: warning: Function parameter or member 'start_pin' not described in 'armada_37xx_pin_group'
 drivers/pinctrl/mvebu/pinctrl-armada-37xx.c:68: warning: Function parameter or member 'val' not described in 'armada_37xx_pin_group'
 drivers/pinctrl/mvebu/pinctrl-armada-37xx.c:68: warning: Function parameter or member 'extra_pin' not described in 'armada_37xx_pin_group'
 drivers/pinctrl/mvebu/pinctrl-armada-37xx.c:68: warning: Function parameter or member 'extra_npins' not described in 'armada_37xx_pin_group'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Gregory Clement <gregory.clement@bootlin.com>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Link: https://lore.kernel.org/r/20200713144930.1034632-25-lee.jones@linaro.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>