ARC: [plat_arcfpga]->[plat_sim]

* Remove remanants of legacy ARC FPGA platforms (AA4, ML509...)
* Only nsim simulation platform is left, rename platform accordingly
* AA4 DT stuff is compatible with nsim for ARC700 so rename it too

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>