default.xml: Update source revision on meta-seco-imx

Update revision to add support for mainline U-Boot's binman tools, USB SDP and Universal Update Utility

Closes #463 (closed), #462 (closed), #464 (closed)

Commit made to meta-seco-imx: https://git.seco.com/pub/i.mx/yocto/5.x/meta-seco-imx/-/commit/2791e673c65d51fc34a89413fd175cfd440b928e

These changes will not yet affect the actual build for seco-imx8mm-c61-2g/4g machines, because the machine overrides, which triggers mainline U-Boot build, is commented out. It is just an intermediate issue MR.