underclock: add a single generic recipe for all watches

[FlorentRevest]

Every watch shipped its own recipes-core/underclock recipe that was identical apart from a device-tuned script. Introduce one generic, machine-independent recipe here so the per-device copies in meta-smartwatch can be dropped.

The script guards every sysfs write behind an existence/writability check, so a single script adapts to whatever a given kernel exposes:

• offline every CPU core except cpu0 (adapts to the core count, so dual-core watches only drop cpu1)
• pick the best available CPU governor: schedutil (mainline since 4.7) when offered, else ondemand -- this reproduces each watch's previous choice from kernel age alone, with no per-device configuration
• pin the GPU to its lowest available frequency, read from kgsl-3d0/devfreq/available_frequencies (no hardcoded MHz)
• enable the lpm_levels / lpm_levels_legacy sleep and idle knobs
• prime MediaTek deep sleep via /dev/spm where present
• apply tetra's PowerHAL fixups (interactive governor + chown of the kona-fb / interactive tunables) only when the tetra-only kona-fb platform device exists

[FlorentRevest]

This goes hand in hand with AsteroidOS/meta-smartwatch#329