i7core_edac: Adds write unlock to MC registers

The public Intel Xeon 5500 volume 2 datasheet describes, on page 53,
session 2.6.7 a register that can lock/unlock Memory Controller the
configuration register, called MC_CFG_CONTROL.

Adds support for it in the hope that software error injection would
work. With my tests with Xeon 35xx, there's still something missing.
With a program that does sequencial bit writes at dev 0.0, sometimes, it
produces error injection, after unblocking the MC_CFG_CONTROL (and,
sometimes, it just locks my testing machine).

I'll try later to discover by trial and error what's the register that
solves this issue on Xeon 35xx.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>