Allow AArch64 processors to boot from a kernel placed over 4GB

Allow AArch64 processors to boot from a kernel placed over 4GB

Architecturally, it's possible for an AArch64 machine to have
all of its RAM over the 4GB mark, but our kernel/initrd loading
code in boot.c assumes that the upper half of the addresses
to load these images to is always zero. Write the whole 64 bit
address into the bootloader code fragment, not just the low half.

Note that, currently, none of the existing QEMU machines have
their main memory over 4GBs, so this was not a user-visible bug.

Signed-off-by: Ricardo Perez Blanco <ricardo.perez_blanco@nokia.com>
[PMM: revised commit message and tweaked some long lines]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>