Currently the compile-time constant PLAT_RK_UART_BASE defines
which UART is used as console device. E.g. on RK3399 it is set
to UART2. That means, that a single bl31 image can not be used
for two boards, which just differ on the UART console.

This patch addresses this limitation by parsing the "stdout-path"
property from the "chosen" node in the DTB. The expected property
string is expected to have the form "serialN:XXX", with
N being either 0, 1, 2, 3 or 4. When the property is found, it will
be used to override PLAT_RK_UART_BASE.

Tested on RK3399-Q7, with a stdout-path of "serial0:115200n8".

Signed-off-by: Christoph Müllner <christophm30@gmail.com>
Change-Id: Iafe1320e77ab006c121f8d52745d54cef68a48c7