[ARM] orion5x: TS-78xx support for 'hotplug' of FPGA devices

the FPGA on the TS-7800 provides access to a number of devices
and so we have to be careful when reprogramming it.  As we
are effectively turning a bus off/on we have to inform the
kernel that it should stop using anything provided by the
FPGA (currently only the RTC however the NAND, LCD, etc is
to come) before it's reprogrammed.

Once reprogramed, we can tell the kernel to (re)enable things
by checking the FPGA ID against a lookup table for what a
particular FPGA bitstream can provide.

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>