s390/boot: Make .amode31 section address range explicit

This is a preparatory rework to allow uncoupling virtual
and physical addresses spaces.

Introduce .amode31 section address range AMODE31_START
and AMODE31_END macros for later use.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>