pinctrl: aspeed: Fix ast2500 strap register write logic

On AST2500, the hardware strap register(SCU70) only accepts write ‘1’,
to clear it to ‘0’, must set bits(write  ‘1’) to SCU7C

Signed-off-by: Yong Li <sdliyong@gmail.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Tested-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>