arm: put the value of VTCR for cells in a define

arm: put the value of VTCR for cells in a define

We can reuse the code under hypervisor/arch/arm/mmu_cell.c for the
AArch64 port, save for the value we use for the VTCRL. AArch64 will
need in addition to the flags set by the AArch32 port, to set the
size of the address space.

We put this behind a define in asm/paging.h to allow this reuse.

Signed-off-by: Antonios Motakis <antonios.motakis@huawei.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>