net: stmmac: intel: only populate plat->crosststamp when supported

To allow the ptp_chardev code to correctly detect whether crosststamps
are supported, we need to conditionally populate the .getcrosststamp()
method. As the previous patch implements that functionality by
detecting whether the platform glue provides a crosststamp() method,
arrange for the dwmac-intel code to only populate this if the X86
ART feature is present, rather than testing for it at runtime in
intel_crosststamp().

This reflects what other x86 PTP clock drivers do, e.g.
ice_ptp_set_funcs_e830(), e1000e_ptp_init(), idpf_ptp_set_caps() etc.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1uto2i-00000001seA-0lxv@rmk-PC.armlinux.org.uk


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