Add support for RockChip RK356X to DWC3 driver.
For RK356x platform, we can set bit 26 of DWC3_GUCTL1 register
for usb 2.0 device.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D36211

(cherry picked from commit 1331c0f44b6a3b6dec1fbd638103fa15bffac5d8)