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>