arm64: dts: ti: k3-am6*-boards: Add label to reserved-memory node

arm64: dts: ti: k3-am6*-boards: Add label to reserved-memory node

Add the label name 'reserved_memory' to the reserved-memory node in all
K3 AM6* board level dts files. This is done so that the node can be
referenced and extended to add more carveout entries as needed in future
refactoring patches.

Signed-off-by: Beleswar Padhi <b-padhi@ti.com>
Acked-by: Andrew Davis <afd@ti.com>
Link: https://patch.msgid.link/20250908142826.1828676-13-b-padhi@ti.com
Signed-off-by: Nishanth Menon <nm@ti.com>
[ upstream commit: 4f1aee4723a796a92f17b23699dc861b582ddfd2 ]

(cherry picked from commit 58c447fe500d78f5adc373b4945d8317e11df072)