riscv32: added support for the SiFive Freedom E310 SOC

The SiFive Freedom E310 SOC follows the riscv privilege
architecture specification and hence is declared within
the riscv privilege SOC family.

It also provides support for a riscv
Platform Level Interrupt Controller (PLIC)

Change-Id: I19ff0997eacc248f48444fc96566a105c6c02663
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>