rpi4: Fix documentation of armstub config entry
rpi4: Fix documentation of armstub config entry

It's in fact mandatory. Seen with RPi firmware 1.20190925.

Signed-off-by: Jan Kiszka <a5b45c9af271d077f30d36aa7d9b988eed9d56eb@siemens.com>
Change-Id: I80739b74f25817294adc52cfd26a3ec59c06f892

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware