serial: pl011: honour serial aliases in device tree

If the order of UART nodes is changed in the device tree, then tty dev
devices are attached to different serial ports causing the console to
be directed to a different physical serial port. The "serial" aliases
in the device tree should prevent this.

This patch ensures that the UART driver creates tty devices that
honour these aliases if a device tree is present.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Matthew Leach <matthew.leach@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>