vchi: rename bitset macros to avoid collision with bitset(9)

An upcoming change to include bitset(9) macros from vm_page.h
causes a macro name collision with vchi's custom bitset macros.

This change was performed mechanically by:

sed -i .orig s/BITSET/VCHI_BITSET/g $(grep -rl BITSET sys/contrib/vchiq)

Reviewed by: andrew
Approved by: scottl (implicit)
MFC after: 1 week
Sponsored by: Ampere Computing, Inc.
Differential Revision: https://reviews.freebsd.org/D26177