gve: implement PTP gettimex64

Enable chrony and phc2sys to synchronize system clock to NIC clock.

Two paths are implemented: a precise path using system counter values
sampled by the device, and a fallback path using system counter values
sampled in the driver using ptp_read_system_prets()/postts().

To use the precise path, the current system clocksource must match the
units returned by the device, which on x86 is X86_TSC and on ARM64 is
ARM_ARCH_COUNTER. The clockid requested for the cross-timestamp must
be either CLOCK_REALTIME or CLOCK_MONOTONIC_RAW. These conditions hold
by default on GCP VMs using Chrony, so we expect the precise path to be
used the vast majority of the time. If the system clocksource is changed
to kvm-clock, it activates the fallback path. Ethtool counters have been
added to count how many times each path is used.

The uncertainty window in the precise path is typically around 1-2us,
while in the fallback path is around 60-80us.

Stub implementions of adjfine and adjtime are added to avoid NULL
dereference when phc2sys tries to adjust the clock.

Cc: John Stultz <jstultz@google.com>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Kevin Yang <yyd@google.com>
Reviewed-by: Naman Gulati <namangulati@google.com>
Signed-off-by: Jordan Rhee <jordanrhee@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20260514225842.110706-4-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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