rockchip: Use coreboot-supplied serial console on coreboot systems

rockchip: Use coreboot-supplied serial console on coreboot systems

This patch changes all Rockchip platforms to initialize the serial
console with information supplied by coreboot rather than hardcoded
base address and divisor values if BL31 is run on top of coreboot.
Moving the BL2-to-BL31 parameter parsing as early as possible to ensure
that the console is available for all following code.

Also update the Rockchip platform to use MULTI_CONSOLE_API.

Change-Id: I670d350fa2f8b8133539f91ac14977ab47db60d9
Signed-off-by: Julius Werner <jwerner@chromium.org>