stmmac: dwmac-socfpga: keep a copy of stmmac_rst in driver priv data

The dwmac-socfpga driver needs to control the reset usually managed
by the core driver to set the PHY mode. Take a copy of the reset
handle from core priv data so it can be used by the driver later.

This also allow us to move reset handling into socfpga_dwmac_setup()
where the code that needs it is located.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Tested-by: Marek Vasut <marex@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>