rockchip: rk3188: rock: adjust for SPL/TPL split

With the changes to split SPL/TPL for the RK3368, I apparently missed
some needed adjustments to the RK3188 Kconfig and rock_defconfig.

This fixes build-issues for the rock board after applying the RK3368
enablement (and SPL/TPL) set that resulted from TPL_SERIAL_SUPPORT,
TPL_ROCKCHIP_BACK_TO_BROM and TPL_TINY_MEMSET being separate symbols
now.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>