target-arm: Fix typo in comment (seperately -> separately)

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Greg Bellows <greg.bellows@linaro.org>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>