i2c: xtensa s6000 i2c driver

Support for the s6000 on-chip i2c controller.

Signed-off-by: Oskar Schirmer <os@emlix.com>
Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>