USB Clock Control
| RESERVED | Reserved. Read value is undefined, only zero should be written. |
| DEV_CLK_EN | Device clock enable. Enables the usbclk input to the device controller |
| RESERVED | Reserved. Read value is undefined, only zero should be written. |
| PORTSEL_CLK_EN | Port select register clock enable. |
| AHB_CLK_EN | AHB clock enable |
| RESERVED | Reserved. Read value is undefined, only zero should be written. |