boards: it8xxx2_evb: fix invalid DTS

The duplicated aliases node in this board's DTS file is an error when
compiling it with dtc. Zephyr's dtlib doesn't report the error yet,
but it will soon, so rearrange the alias definitions under a single
node. No functional changes expected.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>