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

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

Fixes the following W=1 warnings:

 drivers/gpio/gpio-syscon.c:48: warning: Function parameter or member 'compatible' not described in 'syscon_gpio_data'
 drivers/gpio/gpio-syscon.c:48: warning: Function parameter or member 'flags' not described in 'syscon_gpio_data'
 drivers/gpio/gpio-syscon.c:48: warning: Function parameter or member 'bit_count' not described in 'syscon_gpio_data'
 drivers/gpio/gpio-syscon.c:48: warning: Function parameter or member 'dat_bit_offset' not described in 'syscon_gpio_data'
 drivers/gpio/gpio-syscon.c:48: warning: Function parameter or member 'dir_bit_offset' not described in 'syscon_gpio_data'
 drivers/gpio/gpio-syscon.c:48: warning: Function parameter or member 'set' not described in 'syscon_gpio_data'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Alexander Shiyan <shc_work@mail.ru>
Link: https://lore.kernel.org/r/20200630133345.2232932-7-lee.jones@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>