xen/arm: bootfdt: Ignore empty memory bank

xen/arm: bootfdt: Ignore empty memory bank

At the moment, Xen will stop processing the Device Tree if a memory
bank is empty (size == 0).

Unfortunately, some of the Device Tree (such as on Colibri imx8qxp)
may contain such a bank. This means Xen will not be able to boot
properly.

Relax the check to just ignore the banks. FWIW this also seems to be the
behavior adopted by Linux.

Reported-by: Daniel Wagner <Daniel.Wagner2@itk-engineering.de>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit 5a37207df52066efefe419c677b089a654d37afc)