A previous patch already allowed to configure the uart output from the
devicetree, but on Rockchip platforms we also have the issue of different
vendors using different baudrates for their uarts.

For example, rk3399 has a default baudrate of 115200 which is true for
ChromeOS-devices and boards from Theobroma-Systems, while all the boards
using the vendor boot chain actually use a baudrate of 1500000.

Similarly the newly added px30 has a default of said 1500000 but some
boards may want to use the more widely used 115200.

The devicetree stdout-path node already contains the desired baudrate,
so add simple code to parse it from there and override the default,
which stays unchanged.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Change-Id: I7412139c3df3073a1996eb508ec08642ec6af90d