Introduce EFIKA_COMMON

The Genesi EFIKA MX and EFIKA Smartbook are sharing a lot of things
so it makes sense to create a common file for both devices and a specific
file for each. No functionnal change except dropping uart 1 & 2.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>