Change xlate_dev_{kmem,mem}_ptr() prototypes

xlate_dev_mem_ptr() is used to convert a physical address to an uncached
kernel virtual address mapping, so make it use phys_addr_t as type for
the physical address and return void * for the kernel virtual address.

xlate_dev_kmem_ptr() converts a cached kernel virtual address mapping to
an uncached kernel virtual address mapping, so make it use void * for
both the input parameter and return value.

Signed-off-by: Thierry Reding <treding@nvidia.com>