pwm: mediatek: Remove unused function

Fix the following clang warning:

drivers/pwm/pwm-mediatek.c:110:19: warning: unused function
'pwm_mediatek_readl' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>