lib: utils: consider ':' in stdout-path

The value of the /chosen/stdout-path devicetree property is used to
determine the UART used by openSBI. According to the devicetree
specification the value may contain a hyphen, e.g.

	chosen {
                stdout-path = "/serial@f00:115200";
        };

If the character ':' is present, it terminates the path of the device.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>