Merge tag 'arc-fixes-for-2020.04-rc2' of https://gitlab.denx.de/u-boot/custodians...

As usual a bit late a couple of tiny fixes and improvements for ARC.

1. Switch from ARC UART to a convenient DW UART on ARC
   simulation platforms. This became avaialble when nSIM got support
   of that much more standard UART (starting from nSIM v2019.06).
   FWIW also available now in Free nSIM [1].

   This among other things allows us finally to use the same one binary
   on all our simulators & FPGA-based emulators.

2. Disable networking support on simulated platforms as there's no
   network interface in them.

3. Add Virtio net & block devices for the configuration supported
   by QEMU so that we may leverage those virtual peripherals and
   in fact it's possible to load uImage from TFTP server and bootm it.

4. Minor fixes for HSDK clocks.

5. Rework of how we chose and use compiler options for ARC-based boards.
   In real world ARC-based designs are customized more or less but very
   rarely match any of our "templates" thus it makes not much sense to
   pretend we have some fixed configs, instead we now will fully reply
   on a SoC or even board on getting precise set of compiler options
   preferably even extracted from real HW via "tcfgen" utility.

6. Well and finally yet another simplification - switch to generic
   written in C accessors which are much more universal and just work
   for any target supported by the complier as compared to GAS
   implementation which is much more target-dependent.

   This one was heavily "inspired" by similar implementation for RISCV
   and ARM.

[1] https://www.synopsys.com/cgi-bin/dwarcnsim/req1.cgi