printk: ipipe: add raw console channel
A raw output handler (.write_raw) is added to the console descriptor for writing (short) text output unmodified, without any logging, header or preparation whatsoever, usable from any pipeline domain. The dedicated raw_printk() variant formats the output message then passes it on to the handler holding a hard spinlock, irqs off. This is a very basic debug channel for situations when resorting to the fairly complex printk() handling is not an option. Unlike early consoles, regular consoles can provide a raw output service past the boot sequence. Raw output handlers are typically provided by serial console devices.