[POWERPC64LE] fix sysctl dev.opal_sensor.* on little-endian kernel

• fix values returned by 'sysctls dev.opal_sensor.*.sensor'
• fix missing 'dev.opal_sensor.*.sensor_[max|min]' on sysctl

Reviewed-by: jhibbits
Sponsored-by: Eldorado Research Institute (eldorado.org.br)
Differential-Revision: https://reviews.freebsd.org/D27365