usb: dwc3: Cache ref_clk pointer in struct dwc3

Cache ref_clk clock pointer in struct dwc3 . This is a preparatory
patch for subsequent backports from Linux kernel which configure
GFLADJ register content based on the ref_clk rate and therefore need
access to the ref_clk pointer.

It is possible to extract the clock pointer from existing clk_bulk
list of already claimed clock, no need to call clk_get*() again.

Reviewed-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>