doc: ti: Clarify required file names for K3 platforms

Now that we are using binman in all cases on these platforms, reword
things to be clearer that for filesystem booting we need to use a
specific name for each component.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>