[POWERPC] Move iSeries initrd logic into device tree

Remove the iSeries initrd logic, instead just store the initrd location and
size in the device tree so generic code can do the rest for us.

The iSeries code had a "feature" which the generic code lacks, ie. if the
compressed initrd is bigger than the configured ram disk size, we make
the ram disk size bigger. That's bogus, as the compressed size of the initrd
tells us nothing about how big the ram disk needs to be. If the ram disk
isn't big enough you just need to make CONFIG_BLK_DEV_RAM_SIZE larger.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>