gpio: gpio-it87: Fix formatting issues which confuse kerneldoc

Kerneldoc expects struct properties to be documented using the syntax
'@.*: ', but no ':' has been provided in 'struct it87_gpio's
header.  Add them to stop confusing kerneldoc.

Fixes the following W=1 warnings:

 drivers/gpio/gpio-it87.c:69: warning: Function parameter or member 'chip' not described in 'it87_gpio'
 drivers/gpio/gpio-it87.c:69: warning: Function parameter or member 'lock' not described in 'it87_gpio'
 drivers/gpio/gpio-it87.c:69: warning: Function parameter or member 'io_base' not described in 'it87_gpio'
 drivers/gpio/gpio-it87.c:69: warning: Function parameter or member 'io_size' not described in 'it87_gpio'
 drivers/gpio/gpio-it87.c:69: warning: Function parameter or member 'output_base' not described in 'it87_gpio'
 drivers/gpio/gpio-it87.c:69: warning: Function parameter or member 'simple_base' not described in 'it87_gpio'
 drivers/gpio/gpio-it87.c:69: warning: Function parameter or member 'simple_size' not described in 'it87_gpio'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: "Diego Elio Pettenò" <flameeyes@flameeyes.eu>
Link: https://lore.kernel.org/r/20200630133345.2232932-8-lee.jones@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>