arm64: tegra: Prepare for supporting the Jetson Xavier NX with eMMC

There are two versions of the Jetson Xavier NX system-on-module; one
with a micro SD-card slot and one with an eMMC. Currently, only the
system-on-module with the micro SD-card slot is supported. Before adding
support for the eMMC variant, move the common device-tree parts of the
existing Jetson Xavier NX system-on-module board (p3668-0000) and
reference carrier board (p3509-0000) into include files that can be used
by both Jetson Xavier NX variants.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>