[otbn] OTBN autogenerated headers do not have value defines for CMD, STATUS

@GregAC is already working on this.

Since Apr 2, 2024.

See https://github.com/lowRISC/opentitan/pull/16119#discussion_r1043471466

The autogenerated `otbn_regs.h` file does not have defines for field values in the `CMD` or `STATUS` fields. The fields are defined but their values are not enumerated, e.g.:
```c
// Command Register
#define OTBN_CMD_REG_OFFSET 0x10
#define OTBN_CMD_REG_RESVAL 0x0
#define OTBN_CMD_CMD_MASK 0xff
#define OTBN_CMD_CMD_OFFSET 0
#define OTBN_CMD_CMD_FIELD \
((bitfield_field32_t) { .mask = OTBN_CMD_CMD_MASK, .index = OTBN_CMD_CMD_OFFSET })

// Control Register
#define OTBN_CTRL_REG_OFFSET 0x14
...
```

I think this can be fixed by adding an `enum` field to `otbn.hjson` [here](https://github.com/lowRISC/opentitan/blob/7411ac9c6fc12949199bfe70fe43e7fd205c7ef3/hw/ip/otbn/data/otbn.hjson#L326). This seems to produce the desired result for e.g. [KMAC](https://github.com/lowRISC/opentitan/blob/7411ac9c6fc12949199bfe70fe43e7fd205c7ef3/hw/ip/kmac/data/kmac.hjson#L496).

I think I understand what's required here and am happy to make this change myself. But I wanted to check if there's a specific reason these values weren't included. @GregAC do you know?