dt-bindings: timer: Document allwinner,erratum-unknown1 property

This arch_timer property was added in commit c950ca8c
("clocksource/drivers/arch_timer: Workaround for Allwinner A64 timer instability")
but I omitted the device tree binding documentation. Now
`make dtbs_check` prints the following warning for all A64 boards:

timer: 'allwinner,erratum-unknown1' does not match any of the regexes: 'pinctrl-[0-9]+'

Resolve this by documenting the erratum property.
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/timer/arm,arch_timer.yaml

@@ -51,6 +51,12 @@ properties:
     description: If present, the timer is powered through an always-on power
       domain, therefore it never loses context.
 
+  allwinner,erratum-unknown1:
+    type: boolean
+    description: Indicates the presence of an erratum found in Allwinner SoCs,
+      where reading certain values from the counter is unreliable. This also
+      affects writes to the tval register, due to the implicit counter read.
+
   fsl,erratum-a008585:
     type: boolean
     description: Indicates the presence of QorIQ erratum A-008585, which says