adxl345: Add support for adxl345 accelerometer

Add support for taking measurements from an adxl345 accelerometer via
SPI interface.

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

config/example-extras.cfg

@@ -556,6 +556,32 @@
 #   measurements, e.g. with an accelerometer.
 #   Default value is 0.1 which is a good all-round value for most printers.
 
+# Support for ADXL345 accelerometers. This support allows one to query
+# accelerometer measurements from the sensor. This enables an
+# ACCELEROMETER_MEASURE command (see docs/G-Codes.md for more
+# information). The default chip name is "default", but one may
+# specify an explicit name (eg, [adxl345 my_chip_name]).
+#[adxl345]
+#axes_map: x,y,z
+#   The accelerometer axis for each of the printer's x, y, and z axes.
+#   This may be useful if the accelerometer is mounted in an
+#   orientation that does not match the printer orientation. For
+#   example, one could set this to "y,x,z" to swap the x and y axes.
+#   It is also possible to negate an axis if the accelerometer
+#   direction is reversed (eg, "x,z,-y"). The default is "x,y,z".
+#cs_pin:
+#   The SPI enable pin for the sensor. This parameter must be
+#   provided.
+#spi_speed: 5000000
+#   The SPI speed (in hz) to use when communicating with the chip.
+#   The default is 5000000.
+#spi_bus:
+#spi_software_sclk_pin:
+#spi_software_mosi_pin:
+#spi_software_miso_pin:
+#   These optional parameters allow one to customize the SPI settings
+#   used to communicate with the chip.
+
 
 ######################################################################
 # Config file helpers