- main_ram mem_map is now directly used by add_sdram when origin is None.
- max_sdram_size/min_l2_data_width are no longer exposed as targets arguments this can
still be used enforced directly in the few cases it is useful.

boards: Add Vendor prefix to platforms/targets name when useful and when multiple boards from the same vendor. (With Retro-Compat on the imports).

targets: Switch to soc_core_args/soc_core_argdict (instead of soc_sdram that is now deprecated, but still supported for now).

The SoC reset added recently creates a path between sys_clk and pll.clkin
clock domains that is reported by the tools but that can be safely ignored.

targets/pcie: simplify using new LiteX's add_pcie method and enable it on all devices supported by LitePCIe.

targets/xilinx: use generic name for idelay clk (avoid clk200, clk400, etc...) since we somtimes want to change it.

Default to Chaser mode and similar user interface than GPIOOut.

add new board files for VC707 and VCU118, only specified limited ports for VCU118, including clock, reset and DDR4

targets/EthernetSoC: be sure memory region is added before adding Wishbone Slave (required by new SoC)

We initially wanted to provide different level of support for the platforms/targets, mainly
to avoid too much maintenance and let each contributor update its contributed platforms and
targets, but it's easier to update all platforms/targets all-together when LiteX evolves or
changes (and that's what has been done on litex-boards since the creation of the repository).
So let just simplify things and avoid this differentiation.

targets: keep attributes are no longer needed since automatically added when applying constraints to signals.