[U-Boot] [RFC] Serial API cleanup

[Jean-Christophe PLAGNIOL-VILLARD]

Hi,

	currenlty we have 3 api for stdio device : stdio_dev, serial_device
	(multi-serial) and the serial_getc/putc...

	this cleanup will join them in one api the stdio_dev which will support
	the multi-serial for before and after relocation

	this will also pass the current device to the driver to allow it to
	have base address and other information to simplify the implementation
	and avoid workarround as done for ns16550 on the omap3 zoom or ppc4xx
	as example

Best Regards,
J.