arm64: dts: rk3399: improve rock pi 4 emmc config

The issue is that the eMMC reading and writing speeds on Rock Pi 4B is
found to be slow. When testing with a 32G SEC 401 eMMC module, the
reading and writing speeds are 88.9 MB/s and 78.2 MB/s.

According to [0], we can just use the maximum supported frequency value.
According to [1], RK3399 eMMC interface supported HS400, so the option
is added in this change.

After applying this change, testing with a 32G SEC 401 eMMC module on a
Rock Pi 4B:
```
radxa@rock-pi-4b:~$ sudo dd of=/dev/null if=/dev/mmcblk0  status=progress count=6 bs=512M
3221225472 bytes (3.2 GB, 3.0 GiB) copied, 11 s, 295 MB/s
6+0 records in
6+0 records out
3221225472 bytes (3.2 GB, 3.0 GiB) copied, 11.5173 s, 280 MB/s
radxa@rock-pi-4b:~$ sudo dd if=/dev/zero of=/dev/mmcblk0 oflag=direct  status=progress count=6 bs=512M
3221225472 bytes (3.2 GB, 3.0 GiB) copied, 30 s, 107 MB/s
6+0 records in
6+0 records out
3221225472 bytes (3.2 GB, 3.0 GiB) copied, 30.1302 s, 107 MB/s
```

[0]: https://github.com/radxa/kernel/blob/3f671654064b5efa8538748356d2c9bd3b754462/Documentation/devicetree/bindings/mmc/mmc-controller.yaml#L90-L98
[1]: https://opensource.rock-chips.com/images/d/d7/Rockchip_RK3399_Datasheet_V2.1-20200323.pdf

Signed-off-by: Zhaoming Luo <[email protected]>

Author: LZhaoM

arch/arm64/boot/dts/rockchip/rk3399-rock-pi-4.dtsi

@@ -711,9 +711,10 @@
 };
 
 &sdhci {
-	max-frequency = <150000000>;
+	max-frequency = <384000000>;
 	bus-width = <8>;
 	mmc-hs200-1_8v;
+	mmc-hs400-1_8v;
 	non-removable;
 	status = "okay";
 };