linux-firmware: package firmware for Marvell 88W8688

linux-firmware: package firmware for Marvell 88W8688

According to error messages from guruplug

[   35.826441] btmrvl_sdio mmc0:0001:2: Direct firmware load for mrvl/sd8688_helper.bin failed with error -2
[   35.903291] Bluetooth: request_firmware(helper) failed, error code = -2
[   35.909942] Bluetooth: Failed to download helper!
[   36.052820] Bluetooth: Downloading firmware failed!
...
[  764.422739] libertas_sdio: Libertas SDIO driver
[  764.438213] libertas_sdio: Copyright Pierre Ossman
[  764.461186] libertas_sdio mmc0:0001:1: Direct firmware load for libertas/sd8688_helper.bin failed with error -2
[  764.488234] libertas_sdio mmc0:0001:1: Direct firmware load for sd8688_helper.bin failed with error -2
[  764.507382] libertas_sdio: failed to find firmware (-2)

and http://wiki.beyondlogic.org/index.php?title=GuruPlug_Libertas_SD8688,
package the firmware files for Marvell 88W8688, too.

Signed-off-by: Jens Rehsack <sno@netbsd.org>
Signed-off-by: Ross Burton <ross.burton@intel.com>