pwm: mediatek: Update kernel doc for struct pwm_mediatek_of_data

pwm: mediatek: Update kernel doc for struct pwm_mediatek_of_data

The struct never had a member called clk_freq. This fixes the W=1
warning:

	drivers/pwm/pwm-mediatek.c:60: warning: Excess struct member 'clk_freq' description in 'pwm_mediatek_chip'

Fixes: efecdeb82f21 ("pwm: mediatek: Allocate the clks array dynamically")
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240125085649.1571268-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>