rockchip: serial: move driver alias to serial_rockchip.c

rockchip: serial: move driver alias to serial_rockchip.c

The Rockchip uart DT nodes have "snps,dw-apb-uart" as
fall back string. The driver ns16550.c has CONFIG_IS_ENABLED(OF_REAL)
as condition to of_match and does not copy dtplat data.
For TPL/SPL the driver serial_rockchip.c
is used. Move driver alias to correct driver.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>