serial: samsung: add struct s3c24xx_uart_dma

This struct contains DMA configuration for each serial port.
It will be used in next commits adding DMA support in driver.

Based on previous work of Sylwester Nawrocki and Lukasz Czerwinski.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>