Documentation: openrisc: Updates to README

Update the OpenRISC readme to provide some more up-to-date information
on how to get started with OpenRISC.  This includes:

 - remove references to southpole who no longer are consulting for
   OpenRISC (confirmed with Jonas)
 - suggested QEMU instead of the old or1ksim as QEMU is well supported
 - include instructions on how to get an FPGA board running

Suggested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Stafford Horne <shorne@gmail.com>