xen/arm32: head: Mark the end of subroutines with ENDPROC (take two)

xen/arm32: head: Mark the end of subroutines with ENDPROC (take two)

Commit 2ac705a59ef5 ("xen/arm32: head: Mark the end of subroutines
with ENDPROC") intended to mark all the subroutines with ENDPROC.

Unfortunately, I missed fail(), switch_ttbr(), init_uart() and
__lookup_processor_type(). Add ENDPROC for the benefits of
static analysis tools and the reader.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>