tty: amba-pl011: Add earlycon support for SBSA UART

Declare an OF early console for SBSA UART so that the early console device
can be specified via the "stdout-path" property in device-tree.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>