core/cpu: integrate Zynq as a classical CPU (Zynq7000), deprecate SoCZynq.

core/cpu: integrate Zynq as a classical CPU (Zynq7000), deprecate SoCZynq.

This is the logical continuation of the recent change to avoid specific SoC classes.
A Zynq FPGA can be used with or without the PS7. When used without the PS7, a softcore CPU
can be used as with others FPGAs. When using the PS7, the softcore is replaced with the PS7
and connected to the SoC through one of the AXI GP interface.

An example is available on litex-boards.