ARM: imx: eukrea_cpuimx51sd: support rev2 PCB

rev1 were shipped with silicon 2.0, rev2 with silicon 3.0
so we are using the silicon version to know the PCB revision

Signed-off-by: Eric Bénard <eric@eukrea.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>