soft_i2c.c add option for repeated start in i2c_read()

This patch adds a #define to optionally change the behaviour of
i2c_read() in soft_i2c.c to send an I2C repeated start instead of a
stop-start between sending the device address pointer write and
reading back the data.  The current behaviour is retained as the
default.

While most devices will work either way, I have a smart battery(*)
that requires repeated start, and someone at some point found a
device that required a stop-start.

(*) http://www.inspired-energy.com/Standard_Products/NL2054/NL2054%20Rev1.0%20Data%20Sheet.pdf

Signed-off-by: Andrew Dyer <adyer@righthandtech.com>

README

@@ -1509,6 +1509,15 @@ The following options need to be configured:
 		Bus on the MPC8260. But it should be not so difficult
 		to add this option to other architectures.
 
+		CONFIG_SOFT_I2C_READ_REPEATED_START
+
+		defining this will force the i2c_read() function in
+		the soft_i2c driver to perform an I2C repeated start
+		between writing the address pointer and reading the
+		data.  If this define is omitted the default behaviour
+		of doing a stop-start sequence will be used.  Most I2C
+		devices can use either method, but some require one or
+		the other.
 
 - SPI Support:	CONFIG_SPI
 

drivers/i2c/soft_i2c.c

@@ -385,8 +385,18 @@ int  i2c_read(uchar chip, uint addr, int alen, uchar *buffer, int len)
 			}
 			shift -= 8;
 		}
-		send_stop();	/* reportedly some chips need a full stop */
+
+		/* Some I2C chips need a stop/start sequence here,
+		 * other chips don't work with a full stop and need
+		 * only a start.  Default behaviour is to send the
+		 * stop/start sequence.
+		 */
+#ifdef CONFIG_SOFT_I2C_READ_REPEATED_START
 		send_start();
+#else
+		send_stop();
+		send_start();
+#endif
 	}
 	/*
 	 * Send the chip address again, this time for a read cycle.