ARM: dts: STi: Fix aliases property name for STi boards

Update serial aliases from "ttyASN" to more common "serialN".

Since dtc v1.4.6-9-gaadd0b65c987, aliases property name must
be lowercase only. This allows to fix following dtc warnings:

arch/arm/boot/dts/stih418-b2199.dtb: Warning (alias_paths): /aliases: aliases property name must include only lowercase and '-'
arch/arm/boot/dts/stih407-b2120.dtb: Warning (alias_paths): /aliases: aliases property name must include only lowercase and '-'
arch/arm/boot/dts/stih410-b2260.dtb: Warning (alias_paths): /aliases: aliases property name must include only lowercase and '-'
arch/arm/boot/dts/stih410-b2120.dtb: Warning (alias_paths): /aliases: aliases property name must include only lowercase and '-'

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>