riscv32: move riscv privileged architecture specifics within a common header file

Added a riscv-privilege.h header file that contains common
definitions for all riscv SOCs supporting the riscv
privileged architecture specification.

This shall ease addition of future riscv SOCs supporting
the riscv privileged architecture spec.

Change-Id: I5714bf70eeda738a25967ed26d3d0d2aaa0c9989
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>