ARM: dts: am437x: add aliases for all UART instances

Add serialN aliases for all 6 UART instances on
the AM437x SoC so each board's .dts file does not
have to define its own aliases.

Remove the alias added for am437x-gp-evm.dts now
that we have the aliases defined in am4372.dtsi
file.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>