soc/sifive: differentiate between FE310-G000 and FE310-G002

This commit reflects a difference between FE310-G000 and FE310-G002 SoCs,
since only the latter supports PMP. The result of that is the split of the
HiFive1 board into two separate targets, since the HWMv2 right now assumes
that board revisions share the same SoC.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>