serial: Add support for UART on VIA VT8500 and compatibles

This adds a driver for the serial ports found in VIA and WonderMedia
Systems-on-Chip. Interrupt-driven FIFO operation is implemented.
The hardware also supports pure register-based operation (which is
slower) and DMA-based FIFO operation. As the FIFOs are only 16 bytes
long, DMA operation is probably not worth the hassle.

Signed-off-by: Alexey Charkov <alchark@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>