usb: dwc3: add DWC3_GUCTL1 reg for debug

GUCTL1 reg has some useful functions which can be
written by user. For rockchip platform, we set
GUCTL1.DEV_FORCE_20_CLK_FOR_30_CLK (bit26, applicable
for the core is programmed to operate in 2.0 device
only) to 1 in bootrom, and after start the kernel,
we want to check whether this bit can be reset to
default 0 after the core reset. Dump GUCTL1 reg from
debugfs is more convenient for us.

Signed-off-by: William Wu <william.wu@rock-chips.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>