Set KMODDIR to the location of where you want the modules installed:
KMODDIR=${PREFIX}/lib/acpi_perf/
do-install:
@{MKDIR} ${KMODDIR}
cd ${WRKDIR}/modules/acpi_perf ; make install KMODDIR=${KMODDIR}
cd ${WRKDIR}/modules/cpufreq ; make install KMODDIR=${KMODDIR}
cd ${WRKDIR}/modules/powernow ; make install KMODDIR=${KMODDIR}
Scot