ptp: Remove 'default y' for VMCLOCK PTP device

ptp: Remove 'default y' for VMCLOCK PTP device

The VMCLOCK device gives support for accurate timekeeping even across
live migration, unlike the KVM PTP clock. To help ensure that users can
always use ptp_vmclock where it's available in preference to ptp_kvm,
set it to 'default PTP_1588_CLOCK_VMCLOCK' instead of 'default y'.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://patch.msgid.link/89955b74d225129d6e3d79b53aa8d81d1b50560f.camel@infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>