rtw88: 8821c: Add 8821CE to Kconfig and Makefile

Since 8821C code is ready, we can build it.

Signed-off-by: Tzu-En Huang <tehuang@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200603094218.19942-6-yhchuang@realtek.com