arm64: dts: fsl-ls1012a: Fix DTC aliases warnings

Use '-' instead of '_' to fix the following DTC warnings with W=1:

arch/arm64/boot/dts/freescale/fsl-ls1012a-frdm.dtb: Warning (alias_paths): /aliases: aliases property name must include only lowercase and '-'

Cc: Harninder Rai <harninder.rai@nxp.com>
Cc: Bhaskar Upadhaya <Bhaskar.Upadhaya@nxp.com>
Cc: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>