MAINTAINERS: add maintainer for Renesas I2C related drivers

Intentionally missing i2c-riic here, Chris Brandt will add himself for
that one later.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>