Add NXP PCA9685 16-channel 12-bit PWM I2C driver

[benzaid32]

Adds a package for the NXP PCA9685, 16-channel 12-bit PWM I2C LED/servo
driver. This is the chip behind the Adafruit 16-channel PWM/Servo breakout,
so it's a pretty common request for anything that drives multiple hobby
servos or needs more PWM channels than the MCU can offer.

Part used: PCA9685PW,118, TSSOP-28, LCSC C2678753.

What the module exposes

• power (ElectricPower, 2.3V-5.5V, asserted in the module)
• i2c (I2C, default address 0x40 - matches the Adafruit breakout)
• output_enable (ElectricLogic, active-low OE)
• extclk (ElectricSignal, optional external clock)
• led[0:15] (16 ElectricLogic PWM outputs)

Design choices worth calling out

• 10k pull-up on nOE, so outputs default to disabled on power-up. Safer
  for servos than leaving them live while the MCU is still booting.
• 100k pull-down on EXTCLK so the pin isn't floating when the internal
  25MHz oscillator is being used.
• 4.7k I2C pull-ups (Fm+ compatible at 3.3V / 5V).
• 100nF + 10uF decoupling, because 16 outputs switching in phase pulls a
  noticeable current spike.
• 6-bit Addressor on A5:A4:A3:A2:A1:A0 so you can stack up to 62 of
  these on a single bus.

Build status

• ato build passes (default target).
• ato build usage passes (usage example target).
• Tested locally on atopile 0.15.6.
• Only warning during build is LCSC redirecting the datasheet URL to HTML
  instead of a PDF. This happens to several other packages in the repo
  too (it's an LCSC infra quirk, not a code issue).

Notes

• Structure mirrors microchip-mcp23017 and ti-tca9548a: typed
  interfaces, auto-generated parts from ato create part -s C2678753 -a,
  default + usage build targets, MIT license.
• The datasheet names OE active-low (/OE). EasyEDA exports it as OE
  with overbar. The auto-generated part file names the pin nOE and the
  module wires package.nOE to the output_enable interface.

Tested against

NXP datasheet Rev. 4 (16 April 2015). All 28 pin-to-signal mappings
produced by ato create part match the datasheet's Table 3 "Pin
description" for TSSOP-28.