xtensa: enable s6gmac in s6105_defconfig

Signed-off-by: Daniel Glockner <dg@emlix.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Johannes Weiner <jw@emlix.com>
Cc: Oskar Schirmer <os@emlix.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>