Since v2019.06 DesingWare nSIM supports DesignWare UART simulation
and so we may switch from pretty unusual ARC UART to much more standard
DesignWare UART (which in case of U-Boot is just an ordinary 16650 UART).

This among other things makes built dinaries compatible with our other
platforms to name a few: FPGA-based HAPS boards, QEMU and even ZeBU.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>