clk: X1000: Add support for calculat REFCLK of USB PHY.

Add functions for calculat the rate of REFCLK, which is needed by
USB PHY in Ingenic X1000 SoC.

Tested-by: 周正 (Zhou Zheng) <sernia.zhou@foxmail.com>
Signed-off-by: 周琰杰 (Zhou Yanjie) <zhouyanjie@wanyeetech.com>
Link: https://lore.kernel.org/r/20200630163852.47267-4-zhouyanjie@wanyeetech.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>