For whatever reason, I got it working now.
My DT so far:
i2s_tdm: i2s@ff300000 {
compatible = "rockchip,rk3308-i2s-tdm";
reg = < 0x00 0xff300000 0x00 0x1000 >;
interrupts = < 0x00 0x30 0x04 >;
clocks = < 0x02 0x4c 0x02 0x4e 0x02 0xa4 0x02 0x6e 0x02 0x6f 0x02 0x03 0x02 0x04 >;
clock-names = "mclk_tx\0mclk_rx\0hclk\0mclk_tx_src\0mclk_rx_src\0mclk_root0\0mclk_root1";
dmas = < 0x25 0x00 0x25 0x01 >;
dma-names = "tx\0rx";
resets = < 0x02 0x89 0x02 0x8a >;
reset-names = "tx-m\0rx-m";
rockchip,cru = < 0x02 >;
rockchip,grf = < 0x4b >;
rockchip,mclk-calibrate;
pinctrl-names = "default";
pinctrl-0 = < 0x4e 0x4f 0x50 0x51 0x52 0x53 0x54 0x55 0x56 0x57 0x58 0x59 0x5a >;
status = "okay";
rockchip,clk-trm = <1>;
rockchip,bclk-fs = <128>;
#sound-dai-cells = <0>;
phandle = < 0xbe >;
};
tdm_sound {
compatible = "simple-audio-card";
simple-audio-card,name = "rockpis-tdm";
simple-audio-card,format = "dsp_b";
simple-audio-card,mclk-fs = < 128 >;
simple-audio-card,bitclock-master = <&tdm_snd_cpu>;
simple-audio-card,frame-master = <&tdm_snd_cpu>;
status = "okay";
tdm_snd_cpu: simple-audio-card,cpu {
dai-tdm-slot-num = <8>;
dai-tdm-slot-width = <16>;
sound-dai = <&i2s_tdm>;
};
tdm_snd_codec: simple-audio-card,codec {
sound-dai = <&dummy_tdm_codec>;
};
};
dummy_tdm_codec: dummy-codec {
status = "okay";
compatible = "rockchip,dummy-codec";
#sound-dai-cells = < 0x00 >;
phandle = < 0x139 >;
};
I have check that with speaker-test -c 8 -s [1-8] to see that all channels run independently.
Saw on logic analyser:
Clock: 6.144 Mhz
Sync: 48Khz
Only the channels seems to mixed up between speaker-test and the position on tdm. but this can be changed by alsa settings.
Hurra. Now I have drink a box of beer and hopefully radxa pay it 
