hardware but lack the larger fifos rev 5 hardware should have.
The linux world (where our FDT data comes from) solved this by adding
a new property to pl011 nodes, "arm,primecell-periphid". When this
property is present, its values override the values in the hardware
periphid registers. For pl011 rev 5 hardware with small fifos, they
override the id so that it appears to be rev 4 hardware.
The driver now uses the new property when present. It also continues
to check the device compat string, to handle older fdt data that may
still be in use on existing systems (on RPi systems it is common to
update system software without updating fdt data which is part of the
boot firmware).
Reviewed by: imp
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