ARC: nSIM: switch from ARC UART to DW UART

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>