Don't clear HCR_VM bit when updating VTTBR.

Don't clear HCR_VM bit when updating VTTBR.

Currently function p2m_restore_state() would clear HCR_VM bit, i.e.,
disabling stage2 translation, before updating VTTBR register. After
some research and talking to ARM support, I got confirmed that this is not
necessary. We are currently working on a new platform that would need this
to be removed.

The patch is tested on FVP foundation model.

Signed-off-by: Jun Sun <jsun@junsun.net>
Acked-by: Steve Capper <steve.capper@linaro.org>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>