Add Smcsrind/Sscsrind support

This adds the following CSRs:

miselect (0x350), mireg (0x351), mireg2/3 (0x352, 0x353),
mireg4-6 (0x355 - 0x357), siselect (0x150), sireg (0x151),
sireg2/3 (0x152, 0x153), sireg4-6 (0x155 - 0x157), vsiselect (0x250),
vsireg (0x251), mireg2/3 (0x252, 0x253), vsireg4-6 (0x255 - 0x257).

Presently, attempts to read / write from ireg? registers will fail, and
future extensions will provide proxy CSR mappings for the respective
?ireg CSRs.