mcp4451: Add initial support for programming the mcp4451 on lpc176x

Add support for programming smoothieboard current.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example-extras.cfg

@@ -424,6 +424,35 @@
 #   default is to not scale the 'channel_x' parameters.
 
 
+# Statically configured MCP4451 digipot connected via I2C bus (one may
+# define any number of sections with an "mcp4451" prefix).
+#[mcp4451 my_digipot]
+#mcu: mcu
+#   The name of the micro-controller that the MCP4451 chip is
+#   connected to. The default is "mcu".
+#i2c_address:
+#   The i2c address that the chip is using on the i2c bus. This
+#   parameter must be provided.
+#wiper_0:
+#wiper_1:
+#wiper_2:
+#wiper_3:
+#   The value to statically set the given MCP4451 "wiper" to. This is
+#   typically set to a number between 0.0 and 1.0 with 1.0 being the
+#   highest resistance and 0.0 being the lowest resistance. However,
+#   the range may be changed with the 'scale' parameter (see
+#   below). If a wiper is not specified then it is left unconfigured.
+#scale:
+#   This parameter can be used to alter how the 'wiper_x' parameters
+#   are interpreted. If provided, then the 'wiper_x' parameters should
+#   be between 0.0 and 'scale'. This may be useful when the MCP4451 is
+#   used to set stepper voltage references. The 'scale' can be set to
+#   the equivalent stepper amperage if the MCP4451 were at its highest
+#   resistance, and then the 'wiper_x' parameters can be specified
+#   using the desired amperage value for the stepper. The default is
+#   to not scale the 'wiper_x' parameters.
+
+
 # Configure a TMC2130 stepper motor driver via SPI bus. To use this
 # feature, define a config section with a "tmc2130" prefix followed by
 # the name of the corresponding stepper config section (for example,