powernv: Add RF_BIGENDIAN resource flag

The big-endian analogue to the RF_LITTLEENDIAN flag, if this flag is set
on a memory resource it's always mapped big endian.

This is needed by the PowerNV OPAL resources (XIVE and PCI), as these
top-level peripherals are always big-endian.

Tested by: pkubaj
Fixes: c4921a0f("powerpc: Always use BE tag in nexus_map_resource")
MFC after: 1 day
X-MFC-with: c4921a0f