powerpc: Always use BE tag in nexus_map_resource

7b5a5e4e replaced nexus_activate_resource() with the bus generic
equivalent, which uses bus_map_resource(). However, though not intended
to have a functional change, nexus_map_resource() uses the bus tag of
the build (BE vs LE), while nexus_activate_resource() always explicitly
set a big-endian tag, leading to failures on powerpc64le now. Using BE
unless otherwise explicitly specified with the RF_LITTLEENDIAN fixes LE
on bare metal, and doesn't hurt in a VM, which nexus_map_resource() was
added for.

Reported by: pkubaj
Tested by: pkubaj
Fixes: 7b5a5e4e("powerpc nexus: Use bus_generic_rman_*")
MFC after: 1 day