jh7110_pcie.c: add empty PIC methods to conform conventions

This commit updates the driver code to conform with an undocumented
convention which says that certain functions need always be implemented
together regardless of their content (or lack of). It's been said that
unimplemented KOBJ methods become stubs which return ENXIO so this
commit does not imply a functional change.

Reviewed by: mhorne
Differential Revision: https://reviews.freebsd.org/D52042