Add GPIO I2C pHAT

draft/overlay/gpio-i2c-phat.md

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+<!--
+---
+name: GPIO I2C pHAT
+class: board
+type: io
+formfactor: pHAT
+manufacturer: PlasmaDan
+description: 4 Extra I2C Buses for Raspberry Pi.
+github: https://github.com/plasmadancom/GPIO-I2C-pHAT
+buy: https://plasmadan.com/gpioi2c
+image: 'gpio-i2c-phat.png'
+pincount: 40
+eeprom: no
+power:
+ground:
+pin:
+  '4':
+    name: I2C3 SDA
+    mode: i2c
+  '15':
+    name: I2C6 SDA
+    mode: i2c
+  '16':
+    name: I2C6 SCL
+    mode: i2c
+  '29':
+    name: I2C3 SCL
+    mode: i2c
+  '21':
+    name: I2C4 SCL
+    mode: i2c
+  '24':
+    name: I2C4 SDA
+    mode: i2c
+  '32':
+    name: I2C5 SDA
+    mode: i2c
+  '33':
+    name: I2C5 SCL
+    mode: i2c
+-->
+# GPIO I2C pHAT
+
+The GPIO I2C pHAT is an I2C switch for Raspberry Pi which remaps the I2C bus (pins 3 & 5) to 1 of 4 additional buses, allowing any I2C based add-on board to use an alternative I2C bus, without the need for multiplexers or expanders.
+
+Makes use of the extra hardware I2C buses introduced on Raspberry Pi 4 (BCM2711) or software I2C (bit bang). Multiple GPIO I2C boards can be stacked along with other HATs or pHATs. Switching is controlled using a 4-position double-pole slide switch, which allows you to quickly switch between buses.
+
+Includes a breakout header for the 4 extra I2C buses. These are always connected to the associated GPIO pins so can be used regardless of the switch position. Note: external pull-up resistors to 3.3V required.
+
+## Features
+
+* Adds 4 additional I2C buses with breakout
+* User selectable I2C GPIO pin remapping
+* Slide switch allows fast bus switching
+* No-conflict solder jumpers
+* Stackable design
+* Immersion gold plated copper
+
+## Usage
+
+Add required dtoverlay parameter(s) to ```/boot/config.txt``` and reboot.
+
+| Switch Position | Hardware I2C (Pi 4) | Software I2C (Bit Bang) | GPIO |
+| :---: | :---: | :---: | :---: |
+| I2C3 | dtoverlay=i2c3 | dtoverlay=i2c-gpio,bus=3,i2c_gpio_sda=4,i2c_gpio_scl=5 | 4 5 |
+| I2C4 | dtoverlay=i2c4 | dtoverlay=i2c-gpio,bus=4,i2c_gpio_sda=8,i2c_gpio_scl=9 | 8 9 |
+| I2C5 | dtoverlay=i2c5 | dtoverlay=i2c-gpio,bus=5,i2c_gpio_sda=12,i2c_gpio_scl=13 | 12 13 |
+| I2C6 | dtoverlay=i2c6 | dtoverlay=i2c-gpio,bus=6,i2c_gpio_sda=22,i2c_gpio_scl=23 | 22 23 |
+
+Note: When using multiple software I2C buses, add the parameters from highest to lowest, i.e., 6, 5, 4, 3.