ARM: ux500: fix clock for GPIO blocks 6 and 7

The clock assignment in the device tree for GPIO blocks 6
and 7 was incorrect, indicating this was managed by bit 1 on
PRCC 2 while it was in fact bit 11 on PRCC 2.

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