i2c: hisi: Add support to get clock frequency from clock

The clk_rate attribute is not generic device tree bindings for I2C
busses described in Documentation/devicetree/bindings/i2c/i2c.txt.
It can be managed by clock binding.

Support the driver to obtain clock information by clk_rate or
clock property. Find clock first, if not, fall back to clk_rate.

Signed-off-by: Weilong Chen <chenweilong@huawei.com>
Acked-by: Yicong Yang <yangyicong@hisilicon.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>