rp1-ptp-pps: add RP1 Ethernet PHC PPS utility

[ahmadexp]

rp1-ptp-pps adds a self-contained Raspberry Pi utility for the RP1 Ethernet
PHC. It uses the standard Linux PTP interface and testptp, supports selected
BCM GPIOs, and can capture two independent rising-edge EXTTS channels while
producing one PEROUT signal. The package includes testptp, the guarded
command-line runtime, systemd units, an example configuration, and setup
instructions. It does not require raspi-config.

The runtime checks the active PHC's capabilities before mapping pins, prints a
non-actuating plan by default, and requires an explicit --execute for GPIO
operations. The continuous services own and clean their PTP mappings. The
second input channel currently requires active PEROUT because the RP1 timestamp
latch is shared.

The kernel support is proposed separately in
raspberrypi/linux PR 7659.
The utility requires a driver exposing two EXTS channels, one PEROUT channel,
and 28 programmable pins.

Validation:

• Full raspberrypi/utils CMake build succeeds on ARM64.
• The 31 focused runtime tests pass through CTest.
• The compiled testptp -h command succeeds.
• Staged installation includes the executable, runtime scripts, both service
  units, documentation, and example configuration.
• systemd-analyze verify accepts both generated service units.

The simultaneous two-input physical path has not yet been qualified. Earlier
hardware checks exercised input and output sequentially. No board-level
accuracy or jitter guarantee is claimed.