arm64: dts: imx8mm-evk: add pwm1/backlight support

Add pwm1/backlight support nodes for imx8mm_evk board.
Align with u-boot dts

References:
 - https://patchwork.ozlabs.org/project/uboot/patch/20220326111911.13720-9-tommaso.merciai@amarulasolutions.com/



Signed-off-by: Tommaso Merciai <tommaso.merciai@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>