board: ti: am65x: Overwrite get_overlay_mmc

board: ti: am65x: Overwrite get_overlay_mmc

Unlike other K3 SoC's, am65 SoC has the capability to detect daughter
cards and automatically generate a list of white-space separated overlays
in name_overlays environment variable.

When applied during boot with get_overlay_mmc, the path to overlays with
default distribution is incorrect where path is currently: boot/dtb and
the overlays exist in boot/dtb/ti.

Fix the path in get_overlay_mmc so that overlays are automatically applied
correctly during boot time.

Signed-off-by: Judith Mendez <jm@ti.com>