Hi - You can use Resource Governor. But I would recommend to find why cpu usage is high during backup rather than solving it temporarily/ restricting it to certain percentage of cpu.

Is the usage consistently 90+% through out the backup?

Which time are you taking the backup? (Production or off hours)

Which type of backup?

What is the size of database and backup file?

Are you using compression mode to take backup?

What is the waiting types happening during taking the backup?

How many processors are in the server?

What is the physical memory size?

Are you taking the backup to disk/ tape/ network path?

Find out if there is cpu or any other resource bottleneck there.