gpio: gpio-altera: Add missing kerneldoc entry and demote comment

'struct altera_gpio_chip's 'irq_chip' property is undocumented.  So
add property description to the struct's kerneldoc header.  Also
demote comment block which is clearly not in kerneldoc format.

Fixes the following W=1 warnings:

 drivers/gpio/gpio-altera.c:34: warning: Function parameter or member 'irq_chip' not described in 'altera_gpio_chip'
 drivers/gpio/gpio-altera.c:78: warning: Function parameter or member 'd' not described in 'altera_gpio_irq_set_type'
 drivers/gpio/gpio-altera.c:78: warning: Function parameter or member 'type' not described in 'altera_gpio_irq_set_type'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Joyce Ooi <joyce.ooi@intel.com>
Cc: Tien Hock Loh <thloh@altera.com>
Link: https://lore.kernel.org/r/20200630133345.2232932-3-lee.jones@linaro.org


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