pwm: Add MediaTek display PWM driver support

Add display PWM driver support to modify backlight for MT8173 and
MT6595. The PWM has one channel to control the brightness of the
display. When the (high_width / period) is closer to 1, the screen
is brighter; otherwise, it is darker.

Signed-off-by: YH Huang <yh.huang@mediatek.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index 49ef515..0cfaf6b 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig
@@ -249,6 +249,17 @@
 	  To compile this driver as a module, choose M here: the module
 	  will be called pwm-lpss-platform.
 
+config PWM_MTK_DISP
+	tristate "MediaTek display PWM driver"
+	depends on ARCH_MEDIATEK || COMPILE_TEST
+	depends on HAS_IOMEM
+	help
+	  Generic PWM framework driver for MediaTek disp-pwm device.
+	  The PWM is used to control the backlight brightness for display.
+
+	  To compile this driver as a module, choose M here: the module
+	  will be called pwm-mtk-disp.
+
 config PWM_MXS
 	tristate "Freescale MXS PWM support"
 	depends on ARCH_MXS && OF