riscv/sifive: add FE310 Always-on driver

This driver supports SiFive's FE310 Always-on (AON) peripheral's
Real-time clock (RTC) and Watchdog timer (WDT). AON has other
functionality that this driver could support such as the power
management unit (PMU) but that functionality hasn't been implemented.

Reviewed by: philip (mentor), kp (mentor)
Approved by: philip (mentor)
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D24170