rockchip: serial: Kconfig: allow ROCKCHIP_SERIAL enabled in TPL

rockchip: serial: Kconfig: allow ROCKCHIP_SERIAL enabled in TPL

The serial_rockchip.c driver converts platdata to the data structure
used in the ns16550.c file and then calls the function
ns16550_serial_probe().
When compiled with OF_REAL the serial_rockchip.c driver returns
now -ENODEV when probed and does no harm.
The config ROCKCHIP_SERIAL is currently depends on SPL_OF_PLATDATA.
Allow serial port use for both SPL and TPL by removing this
dependency and SPL_BUILD restriction.

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