Macro abyss::mmio

macro_rules! mmio {
    ($(#[$attr:meta])* $N:ident: $($t:tt)*) => { ... };
    ($(#[$attr:meta])* pub $N:ident: $($t:tt)*) => { ... };
    ($(#[$attr:meta])* pub ($($vis:tt)+) $N:ident: $($t:tt)*) => { ... };
}

Make mmio register groups.