usb: dwc3: reference clock period configuration

Set reference clock period when it differs from dwc3 default hardware
set.

We could calculate clock period based on reference clock frequency. But
this information is not always available. This is the case of PCI bus
attached USB host. For that reason we use a custom property.

Tested (USB2 only) on IPQ6010 SoC based board with 24 MHz reference
clock while hardware default is 19.2 MHz.

[ baruch: rewrite commit message; drop GFLADJ code; remove 'quirk-' from
  property name; mention tested hardware ]

[ marek: Ported from Linux kernel commit
         7bee318838890 ("usb: dwc3: reference clock period configuration") ]

Reviewed-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Balaji Prakash J <bjagadee@codeaurora.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Marek Vasut <marex@denx.de> # Port from Linux