rockchip: rk3188: enable TPL_LIBGENERIC for generic memset

Commit c67c8c60 ("board_init.c: Always use memset()") dropped the naive
memset alternative from board_init_f_init_reserve.
So activate CONFIG_TPL_LIBGENERIC for that common memset implementation.
We cannot use the ARCH-specific memset, as that would incur 200bytes of
additional TPL size, space we do not have.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>