net: pcs: Move XPCS into new PCS subdirectory

Create drivers/net/pcs and move the Synopsys DesignWare XPCS into the
new directory. Move the header file into a subdirectory
include/linux/pcs

Start a naming convention of all PCS files use the prefix pcs-, and
rename the XPCS files to fit.

v2:
Add include/linux/pcs

v4:
Fix include path in stmmac.
Remove PCS_DEVICES to avoid new prompts

Cc: Jose Abreu <Jose.Abreu@synopsys.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>