MIPS: Boston: Disable EG20T prefetch

MIPS: Boston: Disable EG20T prefetch

The Intel EG20T Platform Controller Hub used on the MIPS Boston
development board supports prefetching memory to optimize DMA transfers.
Unfortunately for unknown reasons this doesn't work well with some MIPS
CPUs such as the P6600, particularly when using an I/O Coherence Unit
(IOCU) to provide cache-coherent DMA. In these systems it is common for
DMA data to be lost, resulting in broken access to EG20T devices such as
the MMC or SATA controllers.

Support for a DT property to configure the prefetching was added a while
back by commit 549ce8f134bd ("misc: pch_phub: Read prefetch value from
device tree if passed") but we never added the DT snippet to make use of
it. Add that now in order to disable the prefetching & fix DMA on the
affected systems.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21068/
Cc: linux-mips@linux-mips.org