i2c: soft_i2c: add simple GPIO implementation

Since the vast majority of GPIO I2C implementations behave the same way,
support the common GPIO framework with default settings.

This adds two new defines CONFIG_SOFT_I2C_GPIO_{SCL,SDA} so that boards
which want GPIO I2C support need only define these.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Tested-by: Thomas Chou <thomas@wytron.com.tw>
diff --git a/README b/README
index a9c98f2..c9fb284 100644
--- a/README
+++ b/README
@@ -1495,6 +1495,16 @@
 
 		#define I2C_DELAY  udelay(2)
 
+		CONFIG_SOFT_I2C_GPIO_SCL / CONFIG_SOFT_I2C_GPIO_SDA
+
+		If your arch supports the generic GPIO framework (asm/gpio.h),
+		then you may alternatively define the two GPIOs that are to be
+		used as SCL / SDA.  Any of the previous I2C_xxx macros will
+		have GPIO-based defaults assigned to them as appropriate.
+
+		You should define these to the GPIO value as given directly to
+		the generic GPIO functions.
+
 		CONFIG_SYS_I2C_INIT_BOARD
 
 		When a board is reset during an i2c bus transfer