MFC r306376:

Add a sysctl to control the interrupt pacing on AM335x integrated switch.
The hardware can be set to limit the number of interrupts from 2 to 63
interrupts per ms.

To keep the compatibility with the TI documentation the sysctl take the
interval between the interrupts pulses: 16~500 us.

Sponsored by: Rubicon Communications, LLC (Netgate)