xen:arm earlyprintk configuration for Hikey 960 boards

xen:arm earlyprintk configuration for Hikey 960 boards

Introduce an earlyprintk configuration of Hikey 960 boards.

Tested with:
 https://github.com/96boards-hikey/edk2.git #testing/hikey960_v2.5
 https://github.com/96boards-hikey/OpenPlatformPkg.git #testing/hikey960_v1.3.4
 https://git.savannah.gnu.org/git/grub.git #master
 https://github.com/96boards-hikey/linux.git #hikey960-upstream-rebase

For GRUB, the following stanza was used:

GRUB_MODULES="boot chain configfile echo efinet eval ext2 fat font gettext gfxterm gzio help linux loadenv lsefi normal part_gpt par
t_msdos read regexp search search_fs_file search_fs_uuid search_label terminal terminfo test tftp time xen_boot"

grub-install/usr/bin/grub-mkimage \
                --config grub.config \
                --dtb linux/arch/arm64/boot/dts/hisilicon/hi3660-hikey960.dtb \
                --directory=grub/usr/lib64/grub/arm64-efi \
                --output=grubaa64.efi \
                --format=arm64-efi \
                --prefix="/boot/grub" \
                $GRUB_MODULES

And grub.config:
search.fs_label rootfs root

set prefix=($root)/boot/grub
configfile $prefix/grub.cfg

Signed-off-by: Konrad Rzeszutek Wilk <konrad@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <julien.grall@arm.com>