i2c: ocores: add i2c driver for OpenCores I2C controller

Add support for the OpenCores I2C controller IP core
(See http://www.opencores.org/projects.cgi/web/i2c/overview

).

This driver implementation is inspired from the Linux OpenCores
I2C driver available.

Thanks to Peter Korsgaard <peter@korsgaard.com> for writing Linux
OpenCores I2C driver.

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Rick Chen <rick@andestech.com>