hw/riscv/riscv-iommu: Fix process directory table walk

hw/riscv/riscv-iommu: Fix process directory table walk

The PPN field in a non-leaf PDT entry is positioned differently from that
in a leaf PDT entry. The original implementation incorrectly used the leaf
entry's PPN mask to extract the PPN from a non-leaf entry, leading to an
erroneous page table walk.

This commit introduces new macros to properly define the fields for
non-leaf PDT entries and corrects the page table walk.

Signed-off-by: Jason Chien <jason.chien@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250301173751.9446-1-jason.chien@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>