riscv: aw_timer enablement for D1

The SBI timer functionality is partially broken on the Allwinner D1, so
we require this driver to provide an eventtimer/system timer interrupts.

The timecounter interface, on the other hand, is not required. The
generic RISC-V timer driver uses the native rdtime instruction, and
implements vdso, so this should be preferred.

Reviewed by: manu, ganbold
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48672