drivers: disk: nvme: allow building of nvme without mmu

Some SoCs do not have MMUs. Create a new KConfig that `NVME_PRP_PAGE_SIZE`
that will define the PRP page size, and will default to `MMU_PAGE_SIZE` if
there is a MMU. Otherwise, it defaults to 0x1000.

Signed-off-by: Ryan McClelland <ryanmcclelland@meta.com>