modules: gfplatdetect: Add platform detection for imx6qdl

This commit establishes a working module and adds the real code to detect the
platform and to propagate the platform to the /proc/cpuinfo file.
The platform detection module is meant to replace the previous platform
detection code in the imx6 board file.

The kernel, however, needs some adaptions namely the exported symbols for
the system_rev, board_rev, fngboot_rev and macghine_name.
These are defined in arc/arm/kernel/setup.c and are meant to be
displayed via the /proc/cpuinfo file.

The other explicit exports are imx_get_soc_revision and a new function
mxc_get_cpu_type which returns the __mxc_cpu_type variable.

The main parts are taken from the previous implementation in the current
rocko G&F kernel and from the Flash-N-Go Boot board detection code.

Read the "pfid-gpios" from the device tree entry to get the correct
GPIOs. This prevents the need for gpio definitions in this driver.

Backup the previous information in case we want to unload the driver.
Do this mainly for the machine_name string to prevent a dangling
string pointer.

Add another new sysfs interface to read the board and board revision.
This might deprecate the /proc/cpuinfo part so that we do not have to
touch it in the future anymore.
And with teh sysfs we get a consistent method to propagate G&F specific
board information.

Right now only the imx6qdl (San*) platforms are supported. The imx6ull
devices (NALLINO) are not yet supported.