sifive_prci: Add reset support for the FU540 and FU740

This is needed for FU740 PCIe support. Whilst we don't need the FU540's
resets they are also defined for completeness.

Reviewed by: manu
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31024

(cherry picked from commit 8e7e0690ecd79e8adc9182d486c05748bd97d26d)