powerpc: Add RF_LITTLEENDIAN resource flag

If this powerpc-specific flag is set on a resource, then the
little-endian bus tag is always used when mapping that resource.

Make use of this flag in the mpc85xx/fsl_sata driver to avoid setting
the SATA BAR's bus tag after bus_alloc_resource.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D43553