serial: fix pch_uart kconfig & build

The dma_request_channel/dma_release_channel interfaces are not
built when DMADEVICES is not enabled, so make the driver depend on
DMADEVICES.  Also, the help text says that the driver enables & uses
PCH_DMA, which is not enabled, so select that.

ERROR: "__dma_request_channel" [drivers/serial/pch_uart.ko] undefined!
ERROR: "dma_release_channel" [drivers/serial/pch_uart.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Tomoya MORINAGA <tomoya-linux@dsn.okisemi.com>
Reported-by: Zimny Lech <napohybelskurwysynom2010@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>