linux-firmware: add support for Marvell SDIO 8997

linux-firmware: add support for Marvell SDIO 8997

For example, Turris MOX SDIO card is using Marvell (NXP) 88W8997 chip.

Technical specs of 88W8997:
- 28nm
- 802.11 ac wave-2
It should support simultaneous dual-band 2.4 GHz and 5 GHz,
but it requires to support multiSSID for one Wi-Fi card [1], which is
not supported in OpenWrt, yet and if we tried to run two instances of
hostapd, it didn't work well, so it's 2.4 GHz or 5 GHz.
- 2x2 MU-MIMO
- Bluetooth 5.1 with LE support
- Unfortunately, there can be connected only 8 clients at the same time
(limited by FW, however, there exists "enterprise" chip, its equal chip,
it is just different that it uses different FW)

Symlink is necessary as mwifiex_sdio tries to load sd8997_uapsta.bin
[   13.651182] mwifiex_sdio mmc0:0001:1: Direct firmware load for mrvl/sd8997_uapsta.bin failed with error -2
[   13.661065] mwifiex_sdio mmc0:0001:1: Falling back to user helper
[   13.684880] firmware mrvl!sd8997_uapsta.bin: firmware_loading_store: map pages failed
[   13.695910] mwifiex_sdio mmc0:0001:1: Failed to get firmware mrvl/sd8997_uapsta.bin
[   13.703774] mwifiex_sdio mmc0:0001:1: info: _mwifiex_fw_dpc: unregister device

Pali Rohár sent two patches [2] [3] into kernel to fix default firmware name for SD8997, so
the symlink will not be required in the future versions of kernel, which
was accepted and right now, according to my details it was backported to 5.8, 5.7 and 5.4

[1] https://bugs.openwrt.org/index.php?do=details&task_id=3243
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=00eb0cb36fad5
[3] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e1fcac52a9ea

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>