ARM: dts: am43x-epos-evm: Add the am438 compatible string

The SoCs on am43x-epos-evm are named am438x.
Hence add the compatibility string and remove the am4372 string.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>