mcp23017: Add dumps of the microchip mcp23017 i/o expander

[ gsi: minor README markup/flow nits, more perceivable chip function ]

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+-------------------------------------------------------------------------------
+Microchip MCP23017 I/O expander with I2C interface
+-------------------------------------------------------------------------------
+
+This is a set of example captures of the MCP23017 16-bit I/O expander with
+an I2C interface. For details see the MCP23017/MCP23S17 datasheet which is
+titled "16-Bit I/O Expander with Serial Interface":
+
+  http://ww1.microchip.com/downloads/en/DeviceDoc/20001952C.pdf
+
+
+Logic analyzer setup
+--------------------
+
+The logic analyzer used was a Saleae clone (samplerate 1MHz). In addition
+to the I2C datalines, 6 output pins are captured too.
+
+The host was a Raspberry Pi using Python to access the I2C port. The bitrate
+decreased after a few 100ms.
+
+  Probe       MCP23017
+  -------------------
+  0           A0
+  1           A1
+  2           A2
+  3           B0 / A3
+  4           B1 / A4
+  5           B2 / A5
+  6           SDA
+  7           SCL
+
+
+mcp23017_counter_a_write.sr
+---------------------------
+
+Count the Registers OLATA ascending. The lowest 6 bits of the Output A Port
+are captured too (A0 - A5).
+
+
+mcp23017_counter_init_ab_write.sr
+---------------------------------
+
+Reset all registers, then count the registers OLATA ascending and OLATB
+descending. Both registers are set using one word write operation. The
+lowest 3 bits of both output ports are captured too (A0 - A2, B0 - B2).
+
+
+mcp23017_counter_init_ab_write_read.sr
+-------------------------------------
+
+Reset all registers, then count the registers OLATA ascending and OLATB
+descending. Both registers are set and read using one word write operations.
+The lowest 3 bits of both output ports are captured too (A0 - A2, B0 - B2).