rockchip: serial: rename U_BOOT_DRIVER name to rockchip_uart

rockchip: serial: rename U_BOOT_DRIVER name to rockchip_uart

When a defconfig for rk3288 is compiled it gives the warning:

rockchip_rk3288_uart: Missing .compatible in
                      ./drivers/serial/serial_rockchip.c
                    : WARNING: the driver rockchip_rk3288_uart
                      was not found in the driver list

Fix by renaming U_BOOT_DRIVER name of serial_rockchip.c
to rockchip_uart. Add rk3288 serial support with
a DM_DRIVER_ALIAS define.

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