netlink: specs: rt-link: adjust mctp attribute naming

netlink: specs: rt-link: adjust mctp attribute naming

MCTP attribute naming is inconsistent. In C we have:
    IFLA_MCTP_NET,
    IFLA_MCTP_PHYS_BINDING,
         ^^^^

but in YAML:
    - mctp-net
    - phys-binding
      ^
       no "mctp"

It's unclear whether the "mctp" part of the name is supposed
to be a prefix or part of attribute name. Make it a prefix,
seems cleaner, even tho technically phys-binding was added later.

Fixes: b2f63d904e72 ("doc/netlink: Add spec for rt link messages")
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250414211851.602096-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>