ARC: nSIM: DTS: switch UART clock-frequency to 50MHz

UART IP is clocked with 50MHz on HAPS by default. So switch
UART clock-frequency from 100MHz to 50MHz for nsim_* boards
so the binaries can be run on HAPS as well.

This property is dummy in case run in simulator (nSIM) so we
don't need to change anything in nSIM configuration files.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>