net: stmmac: socfpga: add runtime suspend/resume callback for stratix10 platform

According to upstream commit 5ec55823("net: stmmac:
add clocks management for gmac driver"), it improve clocks
management for stmmac driver. So, it is necessary to implement
the runtime callback in dwmac-socfpga driver because it doesn't
use the common stmmac_pltfr_pm_ops instance. Otherwise, clocks
are not disabled when system enters suspend status.

Fixes: 5ec55823 ("net: stmmac: add clocks management for gmac driver")
Cc: stable@vger.kernel.org
Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>