arm64: dts: imx8mp-phyboard-pollux-rdk: Fix led sub-node names

Dtschema of the leds sub-node names expects "led-1" instead of led1.
So, the sub-node names are corrected according to the regexes.

Signed-off-by: Yashwanth Varakala <y.varakala@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>