linux-firmware: add new fw file to ${PN}-rtl8821

Extend the linux-firmware-rtl8821 package with rtw8821 firmware files for
Realtek RTL8821 variants.

Signed-off-by: Bastian Krause <bst@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 23c94593)
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>