ARM: shmobile: Update romImage to relocate appended DTB

Instead of relying of MACH_TYPE for board identification,
update the romImage code to relocate an appended DTB to
the beginning of RAM. This implementation is independent
of ARM_APPENDED_DTB, this because it is necessary to copy
the DTB to memory so the kernel can access it.

Without this patch Mackerel does not boot via the Mask ROM
over USB (r_usb_boot) - this since non-DT boot was broken
ages ago in commit:

0ce53cdc

 ARM: mach-shmobile: Use DT_MACHINE for mackerel

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>