tty/serial: pl011: add DT based earlycon support

Enable DT based earlycon initialization for the pl011 uart.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Acked-by: Grant Likely <grant.likely@linaro.org>