rtw88: 8821c: add dig related settings

To improve user experience in field, we need DIG to adjust RX initial
gain depends on field situation.

Define the register addresses for 8821c.

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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/20200616091625.26489-4-yhchuang@realtek.com