arm_boot: Change initrd load address to "halfway through RAM"

To avoid continually having to bump the initrd load address
to account for larger kernel images, put the initrd halfway
through RAM. This allows large kernels on new boards with lots
of RAM to work OK, without breaking existing usecases for
boards with only 32MB of RAM.

Note that this change fixes in passing a bug where we were
passing an overly large max_size to load_image_targphys()
for the initrd, which meant that we wouldn't correctly refuse
to load an enormous initrd that didn't actually fit into RAM.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Igor Mitsyanko <i.mitsyanko@samsung.com>
Tested-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>