Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/Atmel/ATSAM3A8C/PMC/PMC_SCER#0x0
System Clock Enable Register
Enable USB OTG Clock (48 MHz, USB_48M) for UTMI
Programmable Clock 0 Output Enable
Programmable Clock 1 Output Enable
Programmable Clock 2 Output Enable
https://github.com/cmsis-svd/cmsis-svd-data