PPC: E500: Populate L1CFG0 SPR

When running Linux on e500 with powersave-nap enabled, Linux tries to
read out the L1CFG0 register and calculates some things from it. Passing
0 there ends up in a division by 0, resulting in -1, resulting in badness.

So let's populate the L1CFG0 register with reasonable defaults. That way
guests aren't completely confused.

Reported-by: Shrijeet Mukherjee <shm@cumulusnetworks.com>
Signed-off-by: Alexander Graf <agraf@suse.de>