gpio: 74x164: Allocate buffer with gen_74x164_chip

By moving the internal buffer to the end of struct gen_74x164_chip and
converting it from a pointer to a zero-sized array, it can be allocated
together with gen_74x164_chip, reducing the number of managed
allocations.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>