target-arm: Fix potential buffer overflow

Report from smatch:

target-arm/helper.c:651 arm946_prbs_read(6) error:
 buffer overflow 'env->cp15.c6_region' 8 <= 8
target-arm/helper.c:661 arm946_prbs_write(6) error:
 buffer overflow 'env->cp15.c6_region' 8 <= 8

c7_region is an array with 8 elements, so the index must be less than 8.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
