serial: Get rid of CONFIG_DW_SERIAL

CONFIG_DW_SERIAL is no longer used anywhere so let's forget about it.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Adam Ford <aford173@gmail.com>
Cc: Mario Six <mario.six@gdsys.cc>
Cc: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Cc: Alex Kiernan <alex.kiernan@gmail.com>
Cc: "Jorg Krause" <joerg.krause@embedded.rocks>
Cc: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>