ath79: use the qca,qca9563 chip compatible for the WR818

ath79: use the qca,qca9563 chip compatible for the WR818

All other QCA9563 devices already use this identifier for
the exact SoC. Not that this matters much since as upstream
states in Documentation/devicetree/usage-model.txt:

"First and foremost, the kernel will use data in the DT to
identify the specific machine.  In a perfect world, the
specific platform shouldn't matter to the kernel because all
platform details would be described perfectly by the device
tree in a consistent and reliable manner.

[...]

In the majority of cases, the machine identity is irrelevant,
and the kernel will instead select setup code based on the
machine's core CPU or SoC."

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