tools: ynl-gen: rename basic presence from 'bit' to 'present'

Internal change to the code gen. Rename how we indicate a type
has a single bit presence from using a 'bit' string to 'present'.
This is a noop in terms of generated code but will make next
breaking change easier.

Reviewed-by: David Wei <dw@davidwei.uk>
Link: https://patch.msgid.link/20250505165208.248049-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>