gpio: gpiolib-of: Provide documentation for of_gpio_spi_cs_get_count()

gpio: gpiolib-of: Provide documentation for of_gpio_spi_cs_get_count()

Descriptions for of_gpio_spi_cs_get_count()'s 2 arguments are
missing.  Document both 'dev' and 'con_id'.

Fixes the following W=1 kernel build warnings:

 drivers/gpio/gpiolib-of.c:36: warning: Function parameter or member 'dev' not described in 'of_gpio_spi_cs_get_count'
 drivers/gpio/gpiolib-of.c:36: warning: Function parameter or member 'con_id' not described in 'of_gpio_spi_cs_get_count'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Anton Vorontsov <avorontsov@ru.mvista.com>
Link: https://lore.kernel.org/r/20200630133345.2232932-2-lee.jones@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>