ARM: decompressor: take /memreserve/ entries into account

When trying to decide where to decompress the kernel, we now take DT
memory nodes into account instead of blindly picking a location by
masking the program counter.

Let's refine this another step, by taking /memreserve/ entries into
account as well. This ensures that we don't end up running the kernel
from memory that it should have reserved for platform specific purposes,
and helps us get rid of TEXT_OFFSET hacks in the future.

Note that for now, we only take /memreserve/ entries into account, and
not the /reserved-memory node, which carries a more elaborate
description of the reserved regions. This is fine as these can co-exist,
and it is reasonable to require /memreserve/ entries to be used for this
specific purpose (i.e., marking memory as reserved at the base of DRAM
so that the kernel does not overwrite it before it can be bothered to
check where the memory reservations are).

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>