backlight: ili922x: Add missing kerneldoc description for ili922x_reg_dump()'s arg

Kerneldoc syntax is used, but not complete.  Descriptions required.

Prevents warnings like:

 drivers/video/backlight/ili922x.c:298: warning: Function parameter or member 'spi' not described in 'ili922x_reg_dump'

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Software Engineering <sbabic@denx.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>