s390/boot: initialize control registers in decompressor

Partially revert commit 4555b9f34296 ("s390/boot: move
dma sections from decompressor to decompressed kernel").
This is a prerequisite to allow initialization of virtual
memory in decompressor and avoid overwriting of ASCEs in
the decompressed kernel otherwise.

Since the control registers 2, 5 and 15 are reinitialized
in the decompressed kernel again, this change does not
prevent relocating of amode31 section in any way.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>