coreboot: Add support for CBMEM console

coreboot: Add support for CBMEM console

coreboot supports an in-memory console to store firmware logs even when
no serial console is available. It is widely supported by
coreboot-compatible bootloaders (including SeaBIOS and GRUB) and can be
read by the Linux kernel.

This patch allows BL31 to add its own log messages to this console. The
driver will be registered automatically if coreboot support is compiled
in and detects the presence of a console buffer in the coreboot tables.

Change-Id: I31254dfa0c2fdeb7454634134b5707b4b4154907
Signed-off-by: Julius Werner <jwerner@chromium.org>