ARM: dts: ti: Add AM57xx GP EVM board support

The AM57xx GP EVM boards are built on top the AM57xx BeagleBoard-X15.
The EVM extends the BeagleBoard by adding a touchscreen, some buttons,
and a handful of peripheral extension slots.

Being a plugin extension of an existing standalone board; we define
the am57xx-evm as a composite-DTB of the base am57xx-beagle-x15
and a new am57xx-evm overlay.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Andrew Davis <afd@ti.com>
Message-Id: <20230307161715.15209-2-afd@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>