a3xx: a bit of re-org
a3xx: add RB_MRT registers

Set these up as an <array> of registers..  I think I like this better
than just using <bitset>s for everything.  Will have to see what the
generated headers look like, but I should probably convert other cases
where we have sets of similar registers to use <array> as well, and
inline some of the <bitsets> that are no longer needed.
a3xx: array conversion

Use <array> in more places, where it seems to make sense.
a3xx: more registers