ARM: dts: BCM5301X: Specify serial console params in dtsi files

So far every Northstar device we have seen was using the same serial
console params (115200n8). It probably make the most sense to put it in
some proper dtsi files instead of repeating over and over for every
single device. As different boards may use different bootloaders it
seems the safest idea is to use board specific dtsi files.

Just in case some vendor decides to use different UART (parameters) this
can be always easily overwritten.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>