The SCLK_USB3OTGx_REF clocks is used as reference clock for USB3 block.
Add simple support to get rate of SCLK_USB3OTGx_REF clocks to fix
reference clock period configuration.
Also replace use of 24000000 with the OSC_HZ constant.
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>