ARM: ux500: fix clock for GPIO block 8

The clock assignment in the device tree for GPIO block 8 was
incorrect, indicating this was managed by bit 1 on PRCC 6
while it was in fact bit 1 on PRCC 5.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>