arm64: dts: ls1028a-rdb: enable pwm0

Enable pwm0 on ls1028a-rdb board which uses flextimer1.

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>