AC97 controller registers
`0x00` CSR
[3] (R) Ring Frequency Indicator
[2] (RW) GPIO ena
[1] (RW) AC97_reset_n
[0] (RW) Run
`0x01` Low Level Status
[31] (R) Codec Ready
[28:16] (R) Slot Request (bit 28=Slot 12, bit 16=Slot 0)
[12:0] (R) Slot Valid (bit 12=Slot 12, bit 0=Slot 0)
(Write clears)
`0x02` Codec Register Access
[31] (R) Busy
[30] (RW) Write Enable
[29] (R) Read Error flag
[21:16] (RW) Register Address
[15:0] (RW) Register Value
`0x04` Codec GPIO Input
[19:0] (R) GPIO Input
`0x05` Codec GPIO Output
[19:0] (RW) GPIO Output
`0x06` FIFO Data
[31] (R) Empty flag (for read)
[15:0] (RW) PCM 16 bits signed
`0x07` FIFO Control/Status
[31] (RW) FIFO PCM In - Enable
[30] (RW) FIFO PCM In - Flush
[29] (R) FIFO PCM In - Full
[28] (R) FIFO PCM In - Empty
[27:16] (R) FIFO PCM In - Level
[15] (RW) FIFO PCM Out - Enable
[14] (RW) FIFO PCM Out - Flush
[13] (R) FIFO PCM Out - Full
[12] (R) FIFO PCM Out - Empty
[11:0] (R) FIFO PCM Out - Level
