arch/x86/xen/mmu: Cleanup code/data sections definitions

Cleanup code/data sections definitions
accordingly to include/linux/init.h.

Signed-off-by: Daniel Kiper <dkiper@net-space.pl>
[v1: Rebased on top of latest linus's to include fixes in mmu.c]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>