86xx: Report which "bank" of NOR flash we are booting from on MPC8641HPCN

The MPC8641HPCN board is capable of swizzling the upper address bit of
the NOR flash we boot out of which creates the concept of "virtual"
banks.  This is useful in that we can flash a test of image of u-boot
and reset to one of the virtual banks while still maintaining a
working image in "bank 0".

The PIXIS FPGA exposes registers on LBC which we can use to determine
which "bank" we are booting out of (as well as setting which bank to
boot out of).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>