drivers/virt: pkvm: Add Kconfig dependency on DMA_RESTRICTED_POOL

pKVM guests practically rely on CONFIG_DMA_RESTRICTED_POOL=y in order
to establish shared memory regions with the host for virtio buffers.

Make CONFIG_ARM_PKVM_GUEST depend on CONFIG_DMA_RESTRICTED_POOL to avoid
the inevitable segmentation faults experience if you have the former but
not the latter.

Reported-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>