arm: dts: phycore-am62x: Add missing tifsstub image nodes for FIT loadables

arm: dts: phycore-am62x: Add missing tifsstub image nodes for FIT loadables

The phycore-am62x build was broken due to mkimage reporting an undefined
'image "tifsstub-hs"' in the 'loadables' property of the FIT configuration.
This occurred because the `loadables` field referenced `tifsstub-hs`,
`tifsstub-fs`, and `tifsstub-gp`, but no corresponding nodes were defined
under /images.

This patch was inspired by commit 622f826bf025704cbcc4f39252d4a83129a9cabb
("arm: dts: phycore-am62x: Package TIFS Stub"). It resolves the issue by
adding proper Binman nodes for each TIFS variant (`tifsstub-hs`,
`tifsstub-fs`, and `tifsstub-gp`).

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