powerpc: Fix bootm to boot up again with a Ramdisk

Commit 2a1a2cb6 didnt remove the dummy mem reservation in fdt_chosen,
and this stopped Linux from booting with a Ramdisk. This patch fixes
this, by deleting the useless dummy mem reservation.

When booting with a Ramdisk, a fix offset FDT_RAMDISK_OVERHEAD is now
added to of_size, so we dont need anymore a dummy mem reservation.

I measured the value of FDT_RAMDISK_OVERHEAD on a MPC8270 based
system (=0x44 bytes) and rounded it up to 0x80).

Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Kumar Gala <galak@kernel.crashing.org>