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

Add target for LPDDR4 Test Board

targets/digilent_nexys_video: Add specific Video PLL to give more flexibility on supported Video Timings.

- 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.

ztex213 fix; propagate variant from targets to platform

If useful for several boards, this should probably be provided directly by LiteX.

terasic_deca: Remove --integrated-ram-size parameter (--integrated-main-ram-size provides the same functionnality).

terasic_deca: Remove enforced CPU variant/debug: --cpu-variant=imac or --cpu-variant=imac+debug can be used for this.

The default CPU/Variant is defined in LiteX, enforcing the variant on the target
prevent usage of the other CPUs and also complicate maintenance.

terasic_deca: fix cable name, ulpi, names, add gpio_serial

nexys_video: choose VADJ value with arguments

Signed-off-by: Alessandro Comodi <acomodi@antmicro.com>

targets/lattice_crossing: Avoid direct override of SoCCore.mem_map (break default SoCore.mem_map with updated imports).

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

colorlight_i5: Integrate Video Terminal and Video Framebuffer with ne…