dwc: Rewrite clock and reset functions

dwc: Rewrite clock and reset functions

snps,dwmac have one required clock named stmmaceth and one optional pclk,
correctly handle both in if_dwc, no need to get/enable stmmacseth again
in if_dwc_rk.
It also have one required reset also named stmmaceth and one optional ahb,
correctly handle both.
Rockchip have another optional clock named clk_mac_speed, get it and enable it
if present. Also fix the optional RMII clocks, they were previously wrongly
enabled in RGMII case.