i.MX6 support

[Martin Stein]

Hi Praveen,

I've created a branch
(https://github.com/m-stein/genode/tree/1467_hw_imx6_support) that works
on our Wandboard Quad and our CuBox-i and opened an according issue
(https://github.com/genodelabs/genode/issues/1467). Some annotations:

* I have not merged the Trustzone support that was started in your
branch as there is no scenario for this right now.
* I tried to add the L2-cache support from your branch but it doesn't
work as it is for our boards. Thus, I didn't merge it.
* I modified the kernel to not use the EPIT timer but the CortexA9
Private Timer because it's our default on CortexA9 CPUs and the better
choice when enabling SMP support.
* I've merged some redundant code between i.MX6 and i.MX53 into ``spec/imx``
* I've not merged the uSDHC enums and specs that your i.MX6 port
includes because they should be added by the uSDHC commit

The main differences between our boards and yours seem to be the following:

* the Cortex A9 clock (Wand/Cubox: 792 Mhz, Your board: 800 Mhz) but
this value isn't needed anyway
* the UART instance connected to the serial port (Wand/Cubox:
0x02020000, Your board: 0x021e8000)
* the RAM range configuration

Do you have any objections regarding the commit? If not, I'd suggest you
to re-base your work regarding the i.MX6  onto the branch respectively
the Genode master branch as soon as the commit arrived there to simplify
our collaboration in the future.

Cheers,
Martin