PPC: e500: increase DTC_LOAD_PAD

PPC: e500: increase DTC_LOAD_PAD

An allowance of 5 MiB for BSS is not enough for Linux kernels with certain
debug options enabled (not sure exactly which one caused it, but I'd guess
lockdep).  The kernel I ran into this with had a BSS of around 6.4 MB.

Unfortunately, uImage does not give us enough information to determine the
actual BSS size.  Increase the allowance to 18 MiB to give us plenty of
room.  Eventually this should be more intelligent, possibly packing
initrd+dtb at the end of guest RAM.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>