arm: dts: stm32mp15: remove duplicate uart nodes

Remove duplicated uart nodes introduced with commit 62f95af9
("ARM: dts: stm32mp1: DT alignment with Linux kernel v5.9-rc4"),
because the uart nodes wasn't correctly ordered in alphabetic order.

Only cosmetic: the generated device tree don't change.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>