of: earlycon: Add options string handling

Pass-through any options string in the 'stdout-path' property to the
earlycon "driver" setup.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>