Align the device tree blob to a 4KB boundary, not to QEMU's
idea of a page boundary -- the latter is the smallest possible
page size for the architecture, which on ARM is 1KB.
The documentation for Linux does not impose separation
or alignment requirements on the device tree blob, but
in practice some kernels will happily trash the entire
page the initrd ends in after they have finished uncompressing
the initrd. So 4KB-align the DTB to ensure it does not get
trampled by these kernels.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>