ARC: [nsimosci] Change .dts to use generic 8250 UART

The Synopsys APB DW UART has a couple of special features that are not
in the System C model. In 3.8, the 8250_dw driver didn't really use these
features, but from 3.9 onwards, the 8250_dw driver has become incompatible
with our model.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: <stable@vger.kernel.org>   #3.10, 3.12, 3.13, 3.14
Cc: Francois Bedard <Francois.Bedard@synopsys.com>