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>