pwm: mediatek: Provide missing kerneldoc description for 'soc' arg

Kerneldoc syntax is used, but not complete.

Descriptions are required for all arguments.

Fixes the following W=1 build warning:

 drivers/pwm/pwm-mediatek.c:57: warning: Function parameter or member 'soc' not described in 'pwm_mediatek_chip'

Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: John Crispin <blogic@openwrt.org>
Cc: Zhi Mao <zhi.mao@mediatek.com>
Cc: linux-pwm@vger.kernel.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>