target/ppc: add vhyp addressing mode helper for radix MMU

target/ppc: add vhyp addressing mode helper for radix MMU

The radix on vhyp MMU uses a single-level radix table walk, with the
partition scope mapping provided by the flat QEMU machine memory.

A subsequent change will use the two-level radix walk on vhyp in some
situations, so provide a helper which can abstract that logic.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20220216102545.1808018-5-npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>