ARM: dts: imx6qdl-nit6xlite: use hyphens for nodes name

Therefore aligning the panel nodes name across all platforms.

Also removing the bt_rfkill node since the mainline rfkill-gpio driver
doesn't support device trees.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>