Option to use raw digipot values

[grauerfuchs]

For machines with the MCP4018 digipot controller (i.e. Mightyboard)

The existing driver hard-codes the current calculations for the A4988 stepper driver. This is fine on stock machines, but users must fudge the numbers if they use a different type of stepper driver on one or more axes.

Usage:
Defining the new MCP4018_USE_RAW_VALUES config option will enable use of the DIGIPOT_MOTOR_CURRENT config option values rather than DIGIPOT_I2C_MOTOR_CURRENTS, and M907 will accept raw wiper values (0-127) on all channels.

Testing:
This code is now running on both of my FFCP machines with TMC2100s on X and Y and the original steppers on Z, A, and B. The TMC2100 driver modules in use are designed specifically for drop-in compatibility on the Mightyboard, including use of the digipot for setting Vref.

[tharts]

This change seems to break building for MKS Sbase (LPC1768) if mcp4451 is uncommented. Sanity check complains about missing pins definitions which wasn't the case before.
Digipots worked with the previous version when the option digipot_i2c was uncommented.

[thinkyhead]

Patched! Apparently MKS SBASE is special and provides its own interface. Others will still need these pins.

[tixenre]

This change seems to break building for MKS Sbase (LPC1768) if mcp4451 is uncommented. Sanity check complains about missing pins definitions which wasn't the case before.
Digipots worked with the previous version when the option digipot_i2c was uncommented.

I'm having the same issue building for Smoothieboard (LPC1769) if mcp4451 on Marlin 2.06
#error "DIGIPOT_MCP4018/4451 requires DIGIPOTS_I2C_SDA_* pins to be defined."

[thinkyhead]

Comment out the #error line and see if it can build.