Move pcs-xpcs kmod from armsr modules.mk to generic modules package.
Also add additional dependency to x86_64 as stmmac-core it's now used
by x86_64 target and depends on this package.
Fixes: 4b4c940fbc8f ("x86: Add kmod-dwmac-intel")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>