powerpc: Fix device tree padding associated with ramdisk

powerpc: Fix device tree padding associated with ramdisk

When booting with a ramdisk we bump the amount of memory reserved for
the device tree by FDT_RAMDISK_OVERHEAD.  However we did not increase
the actual size in the device tree blob to match.

Its possible on boundary cases that we dont have enough memory according
to the device tree blob and get errors like:

WARNING: could not set linux,initrd-end FDT_ERR_NOSPACE

We can easily fix this by setting the device tree size at the same time
we bump the amount of memory reserved for the device tree.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>