riscv32: added the riscv-privilege SOC_FAMILY

added the riscv-privilege SOC_FAMILY, under which all
riscv SOCs supporting the riscv privilege architecture
specifcation shall reside. These SOCs shall notably have
a common base for handling IRQs.

Moved riscv32-qemu under the riscv-privilege SOC_FAMILY

Change-Id: I5372cb38e3eaed78886f22b212ab4f881ef30b3f
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>