mcp4728: Add initial support for the mcp4728 i2c dac chip

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

config/example-extras.cfg

@@ -639,6 +639,36 @@
 #   to not scale the 'wiper_x' parameters.
 
 
+# Statically configured MCP4728 digital-to-analog converter connected
+# via I2C bus (one may define any number of sections with an "mcp4728"
+# prefix).
+#[mcp4728 my_dac]
+#i2c_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. The default
+#   is zero.
+#channel_a:
+#channel_b:
+#channel_c:
+#channel_d:
+#   The value to statically set the given MCP4728 channel to. This is
+#   typically set to a number between 0.0 and 1.0 with 1.0 being the
+#   highest voltage and 0.0 being the lowest voltage. However, the
+#   range may be changed with the 'scale' parameter (see below). If a
+#   channel is not specified then it is left unconfigured.
+#scale:
+#   This parameter can be used to alter how the 'channel_x' parameters
+#   are interpreted. If provided, then the 'channel_x' parameters
+#   should be between 0.0 and 'scale'. This may be useful when the
+#   MCP4728 is used to set stepper voltage references. The 'scale' can
+#   be set to the equivalent stepper amperage if the MCP4728 were at
+#   its highest voltage, and then the 'channel_x' parameters can be
+#   specified using the desired amperage value for the stepper. The
+#   default is to not scale the 'channel_x' parameters.
+
+
 # Configure an SX1509 I2C to GPIO expander. Due to the delay incurred
 # by I2C communication you should NOT use SX1509 pins as stepper enable,
 # step or dir pins or any other pin that requires fast bit-banging. They