gpio: add support for FTDI's MPSSE as GPIO

gpio: add support for FTDI's MPSSE as GPIO

FTDI FT2232H is a USB to GPIO chip. Sealevel produces some devices
with this chip. FT2232H presents itself as a composite device with two
interfaces (each is an "MPSSE"). Each MPSSE has two banks (high and low)
of 8 GPIO each. I believe some MPSSE's have only one bank, but I don't
know how to identify them (I don't have any for testing) and as a result
are unsupported for the time being.

Additionally, this driver provides software polling-based interrupts for
edge detection. For the Sealevel device I have to test with, this works
well because there is hardware debouncing. From talking to Sealevel's
people, this is their preferred way to do edge detection.

Signed-off-by: Mary Strodl <mstrodl@csh.rit.edu>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20241009131131.1618329-1-mstrodl@csh.rit.edu
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>