ARM: dts: imx: replace underscore with hyphen in aliases name

It replaces underscore with hyphen in aliases name to fix DTC
alias_paths warning below, which is seen with various i.MX board dts
files when W=1 switch is on.

 Warning (alias_paths): /aliases: aliases property name must include only lowercase and '-'

Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Acked-By: Lothar Waßmann <LW@KARO-electronics.de>
Reviewed-by: Gary Bisson <gary.bisson@boundarydevices.com>