apm821xx: fix sata access freezes

The original vendor's driver programmed the dma controller's
AHB HPROT values to enable bufferable, privileged mode. This
along with the "same priorty for both channels" fixes the
freezes according to @takimata, @And.short, that have been
reported on the forum by @ticerex.

Furtheremore, @takimata reported that the patch also improved
the performance of the HDDs considerably:
|<https://forum.lede-project.org/t/wd-mybook-live-duo-two-disks/16195/55>
|It seems your patch unleashed the full power of the SATA port.
|Where I was previously hitting a really hard limit at around
|82 MB/s for reading and 27 MB/s for writing, I am now getting this:
|
|root@OpenWrt:/mnt# time dd if=/dev/zero of=tempfile bs=1M count=1024
|1024+0 records in
|1024+0 records out
|real    0m 13.65s
|user    0m 0.01s
|sys     0m 11.89s
|
|root@OpenWrt:/mnt# time dd if=tempfile of=/dev/null bs=1M count=1024
|1024+0 records in
|1024+0 records out
|real    0m 8.41s
|user    0m 0.01s
|sys     0m 4.70s
|
|This means: 121 MB/s reading and 75 MB/s writing!
|
|[...]
|
|The drive is a WD Green WD10EARX taken from an older MBL Single.
|I repeated the test a few times with even larger files to rule out
|any caching, I'm still seeing the same great performance. OpenWrt is
|now completely on par with the original MBL firmware's performance.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

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