o Grab the number of devices supported by PLIC from FDT.
o Fix bug in PLIC_ENABLE macro when irq >= 32.

Tested on the real hardware, which is HiFive Unleashed board.

Thanks to SiFive, Inc. for the board provided.

Reviewed by: markj
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D19775