gpio: gpio-it87: Add support for IT8620 and IT8628

These chips seem to have a 9th GPIO block (thus supporting 72 GPIOs)
which is configured through SuperIO register 0xd2 (output enable) and
0xd3 (simple I/O). This is also the reason why io_size is larger than
on IT8728 / IT8732. Unfortunately I don't have hardware to test this 9th
GPIO block.

I am also not sure about not configuring the Simple I/O registers as the
hardware I have only uses GPIO block 8. Reading back the values of
0xc0-0xc7 (as configured by the BIOS/EFI on my board) shows that all
have 0xff set.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>