arm64: dts: ls1028a: add flextimer based pwm nodes

Add pwm nodes using flextimer controller.

Signed-off-by: Biwen Li <biwen.li@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>