MAINTAINERS: add entry for i2c-axxia driver

Create separate entry for i2c-axxia and set myself as maintainer.

Even though I don't work for Intel/LSI who is the manufacturer of this
IP, I do have access to some documentation and the actual hardware using
this driver and I'm familiar with the code.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>