Claude review: drm/tyr: add MMU address space registers

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Author:** Daniel Almeida

Adds register definitions and an `AsRegister` helper for per-address-space register access.

One concern: `AsRegister::new()` does not validate `as_nr`:

```rust
+impl AsRegister {
+    fn new(as_nr: usize, offset: usize) -> Result<Self> {
+        Ok(AsRegister(mmu_as(as_nr) + offset))
+    }
```

It returns `Result` but never returns an error. If `as_nr >= MAX_AS_REGISTERS`, the computed offset will be out of the expected MMIO range. While callers in the address_space module do call `validate_as_slot()` before using these registers, the validation is in `AsRegister::new` is misleading since it always succeeds. Consider either adding a bounds check here or changing the return type to plain `AsRegister` (not `Result`). Given the `try_read32`/`try_write32` calls in read/write will catch truly out-of-bounds accesses at the IoMem level, this is a defense-in-depth concern rather than a functional bug.

Minor nit: `AS_MEMATTR_AARCH64_SH_MIDGARD_INNER` is `0 << 4` which is just `0`. A comment noting this is intentional (the zero value selects "midgard inner" shareability) would improve readability.

The change from `MAX_AS_REGISTERS = 32` to `16` matches the Mali CSF spec — good fix.

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