ARM: ux500: 8500: use hsit_a_2 group for HSI

Due to specific constraint, pin HSIT_ACWAKE0_a is
not driven by HSI block but as a GPIO.

Cc: arm@kernel.org
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Patrice Chotard <patrice.chotard@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>