Correct references to i2c-mux.txt which was previously mux.txt.

Also correct the spelling of relevant.
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>
Acked-by: NPeter Rosin <peda@axentia.se>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>