tty: serial: bcm63xx: Enable DT earlycon support

This enables early console output if there is a chosen/stdout-path
property referencing a UART node with the "brcm,bcm6345-uart" compatible
string.  The bootloader sets up the pinmux and baud/parity/etc.
Tested on bcm3384 (MIPS, DT).

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>