configs/ti_am62x_sk: bump U-Boot and Linux versions

This patch bumps:

- U-Boot to version v2025.07
- Linux kernel to version 6.12.36 (LTS)

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>