bcm2835-dma: suppress warning

Suppress the following warning by casting the pointer to and uintptr_t
before to u32:

  drivers/dma/bcm2835-dma.c: In function ‘bcm2835_dma_probe’:
  drivers/dma/bcm2835-dma.c:1179:16: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
    chan_start = ((u32)base / BCM2835_DMA_CHAN_SIZE) & 0xf;
                  ^

Signed-off-by: Matteo Croce <mcroce@redhat.com>