tty: serial: bcm63xx: Add support for unnamed clock outputs from DT

The original non-DT bcm63xx clk code ignores the struct device argument
and looks up a global clock name.  DT platforms, by contrast, often just
use a phandle to reference a clock node with no "clock-output-names"
property.  Modify the UART driver to support both schemes.

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