[malloc] Rename malloc_dma() to malloc_phys()

The malloc_dma() function allocates memory with specified physical
alignment, and is typically (though not exclusively) used to allocate
memory for DMA.

Rename to malloc_phys() to more closely match the functionality, and
to create name space for functions that specifically allocate and map
DMA-capable buffers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/malloc.c

@@ -596,8 +596,8 @@ void * malloc ( size_t size ) {
  *
  * @v ptr		Memory allocated by malloc(), or NULL
  *
- * Memory allocated with malloc_dma() cannot be freed with free(); it
- * must be freed with free_dma() instead.
+ * Memory allocated with malloc_phys() cannot be freed with free(); it
+ * must be freed with free_phys() instead.
  *
  * If @c ptr is NULL, no action is taken.
  */