[FR] M260/M261 i2c command register support

[nvtkaszpir]

Is your feature request related to a problem? Please describe.

Right now M260 and M261 allows writing to i2c address in a basic form, but a lot of devices are much more easy to operate when specifying command register to read/write values to it.

Are you looking for hardware support?

Stock Prusa Mini+

Describe the feature you want

A lot of i2c devices specify command registers they use for certain read/write operations, which makes using them a bit easier with other libraries, for example smbus.

Example writing to PCF8591 (I know it is ancient) under linux using i2c command:
device address: 0x48
command register: 0x40 - this an analog out pin available on the given module
value: 0x0

i2cset -y 1 0x48 0x40 0x0

or set it to 255

i2cset -y 1 0x48 0x40 0xff

Also please see https://www.waveshare.com/wiki/Raspberry_Pi_Tutorial_Series:_PCF8591_AD/DA for example code

Additional context

I saw https://www.printables.com/model/1045824-prusa-mini-stacklight which uses PCA9554, but I wanted to use PCF8591.
I thing C in gcode command could be used for 'command' (because R is already used, but maybe it would be useful as 'register' but that would be breaking experimental feature).