fix ethernet clock (it's a GMII), add FB support over the HDMI connector (hdmi clock set from the resolution)

- 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.

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

Collect --with-ethernet/--with-etherbone, --with-spi-sdcard/--with-sdcard,
etc. into ArgumentParser.add_mutually_exclusive_group()s.  That way, we
get pretty --help output, and appropriate error messages if somebody
tries to ask for something that doesn't make sense.

Build tested with /qmtech_wukong.py --with-sdcard --with-ethernet --integrated-rom-size=0x10000 --build.