Add field access types to MMIO

Update MMIO and regz to handle and generate code like: ``` const Register = MMIO(packet struct(u32) { field1: u17 = 0x5, field2: u15 = 0xAB, }, .{ .field1 = .read_only, .field2 = .write_only, }); ``` We can create compile-time errors for writing to read-only fields, and vise-versa.