dt-bindings: sp804: add support for Hisilicon sp804 timer

Some Hisilicon SoCs, such as Hi1212, use the Hisilicon extended sp804
timer.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20200919124412.4135-2-thunder.leizhen@huawei.com


Signed-off-by: Rob Herring <robh@kernel.org>