xen/arm64: Make sure we get all debug output

xen/arm64: Make sure we get all debug output

Starting in the wrong ELx mode I get the following debug output:

...
- Current EL 00000004 -
- Xen must be entered in NS EL2 mode -
- Boot failed -

The output of "Please update the bootloader" is missing here, because
string concatenation in gas, unlike in C, keeps the \0 between each
individual string.

Make sure this is output, too. With this, we get

...
- Current EL 00000004 -
- Xen must be entered in NS EL2 mode -
- Please update the bootloader -
- Boot failed -

as intended.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- added same change to arm32 case ]