Documentation for OpenRISC port

OpenRISC architecture specification:

 https://raw.githubusercontent.com/openrisc/doc/master/openrisc-arch-1.3-rev1.pdf



Currently the port as of the 2022-01-03 rebasing there are no known
architecture specific test failures.

Writing credits for the port are:

 Stafford Horne  <shorne@gmail.com>
 Christian Svensson  <blue@cmd.nu>

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>