riscv: define mvendorid CSR with macro helpers

[rmsyn]

Uses CSR macro helpers to define the mvendorid CSR register.

[romancardenas]

Should we add the jedec_manufacturer field?

[rmsyn]

Good question, I missed that function when converting to the CSR macro.

After reading the ISA again, it looks like the jedec_manufacturer implementation was incorrect.

mvendorid encodes the number of one-byte continuation codes in the Bank field, and
encodes the final byte in the Offset field, discarding the parity bit. For example, the JEDEC
manufacturer ID 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x8a (twelve
continuation codes followed by 0x8a) would be encoded in the mvendorid CSR as 0x60a.

So, we would probably want to add bank and offset fields, along with a correct jedec_manufacturer function that decodes the ID:

pub const fn jedec_manufacturer(&self) -> (usize, usize) {
    (self.bank(), 0x80 | self.offset())
}