sun4i-emac.c: add dma support

Thanks for your review. Here is the new version for this patch.

This patch adds support for the emac rx dma present on sun4i. The emac
is able to move packets from rx fifo to RAM by using dma.

Change since v4.
  - rename sbk field to skb
  - rename alloc_emac_dma_req to emac_alloc_dma_req
  - using kzalloc(..., GPF_ATOMIC) in interrupt context to avoid
    sleeping
  - retry by using emac_inblk_32bit when emac_dma_inblk_32bit fails
  - fix some code style issues

Change since v5.
  - fix some code style issue

Signed-off-by: Conley Lee <conleylee@foxmail.com>
Link: https://lore.kernel.org/r/tencent_DE05ADA53D5B084D4605BE6CB11E49EF7408@qq.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>