alpha: Convert mapping routine to rely on physical address

alpha: Convert mapping routine to rely on physical address

Alpha doesn't need struct *page and can perform mapping based on
physical addresses. So convert it to implement new .map_phys callback.

As part of this change, remove useless BUG_ON() as DMA mapping layer
ensures that right direction is provided.

Tested-by: Magnus Lindholm <linmag7@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20251015-remove-map-page-v5-7-3bbfe3a25cdf@kernel.org