rtw88: 8723d: Add 8723DE to Kconfig and Makefile

Since 8723D code is ready, we can build it.

Signed-off-by: Ping-Ke Shih <pkshih@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/20200512102621.5148-10-yhchuang@realtek.com