powerpc: Get rid of bitfields in ppc_bat struct

While working on the 36-bit physical support, I noticed that there
was exactly one line of code that actually referenced the bitfields.
So I got rid of them and redefined ppc_bat as a struct of 2 u32's:
batu and batl.  I also got rid of the previous union that held the
bitfield structs and a word representation of the batu/l values.

This seems like a nicer solution than adding in a bunch of
new bitfields to support extended bat addressing that would never
get used, and just leaving the struct as-is would have been
incomplete in the face of large physical addressing.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>