net: stmmac: create dwmac-intel.c to contain all Intel platform

As stmmac_pci.c file is getting bigger and more complex, it is reasonable
to separate all the Intel specific dwmac pci device to a different file.
This move includes Intel Quark, TGL and EHL. A new kernel config
CONFIG_DWMAC_INTEL is introduced and depends on X86. For this initial
patch, all the necessary function such as probe() and exit() are identical
besides the function name.

Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>