The SCLK_USB3OTG_REF clocks is used as reference clock for USB3 block.

Add simple support to get rate of SCLK_USB3OTG_REF clocks to fix
reference clock period configuration.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>