rnndec: add rnndec_decode_enum

Needed to decode pm4 packet ids.
adreno: CP reg cleanup

A lot more seems to be in common between a3xx and a2xx, judging by
comparing which registers are written by the ME for the parts of the fw
which handle various PM4 packets for a3xx vs a2xx firmware.
adreno: add internal PM4 packet names

From amd-gpu headers.. not 100% sure they are same for a3xx, but they do
seem to be so far.
a2xx: couple more regs from amd-gpu headers

Registers that I hadn't seen before in cmdstream, but which are written
by the ME firmware.
a3xx: add some registers found in firmware