rockchip: rk3399: enable SPL_SERIAL_SUPPORT and SPL_DRIVERS_MISC_SUPPORT via Kconfig

SPL_SERIAL_SUPPORT and SPL_DRIVERS_MISC_SUPPORT were previously
enabled through rk3399_common.h.  This change implies these options
through Kconfig.

These need to always be active for the RK3399, as follows:
 - SPL_SERIAL_SUPPORT is needed to pass the SPL build
 - SPL_DRIVERS_MISC_SUPPORT is needed to pass the SPL build

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>