arm64: xen: Use modern annotations for assembly functions

In an effort to clarify and simplify the annotation of assembly functions
in the kernel new macros have been introduced. These replace ENTRY and
ENDPROC. Update the annotations in the xen code to the new macros.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Julien Grall <julien@xen.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>