rtw88: add legacy firmware download for 8723D devices

The WLAN CPU of 8723D device is different from others, add legacy
firmware download function for it. A new variable wlan_cpu is used to
decide which firmware download function we should use.

Legacy firmware file contains 32 bytes header including version and
subversion. When downloading to wlan cpu, header is excluded.

Firmware is downloaded via beacon queue to reserved page that is a part of
TX buffer. Since 11N WLAN CPU uses different control registers, this patch
introduces related control registers.

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/20200422034607.28747-2-yhchuang@realtek.com