arm: k3-am65-iot2050: Fix invalid default configuration reference

arm: k3-am65-iot2050: Fix invalid default configuration reference

mkimage introduced a validation step to ensure that the 'default'
property under the /configurations node in a FIT image refers to a
valid subnode. This exposed an issue in the
k3-am65-iot2050-boot-image.dtsi, where the 'default' property was set to
"ti/k3-am6528-iot2050-basic" and "ti/k3-am6528-iot2050-basic-pg2", which
do not correspond to actual configuration node names.

This patch updates both /configurations nodes to reference the correct
subnode.

Without this fix, mkimage will fail with an error:
"ERROR: Default configuration 'ti/k3-am6528-iot2050-basic' not found
under /configurations"

An upcoming commit will validate the if the default configuration exists

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>