firmware: add support for PCMCIA Aircard's

Add .cis firmware for Sierra Wireless Aircard GPRS/3G PCMCIA modems:

 - Aircard 555
 - Aircards 7xx
 - Aircards 8x0

There is a package named sierra-pcmcia in obsoleted repository for
packages at git.openwrt.org but there's no reason to reintroduce it in
current packages repo because these files are part of upstream
linux-firmware.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>