gpio: idio-16: Migrate to the regmap API

The regmap API supports IO port accessors so we can take advantage of
regmap abstractions rather than handling access to the device registers
directly in the driver.

By leveraging the regmap API, the idio-16 library is reduced to simply a
devm_idio_16_regmap_register() function and a configuration structure
struct idio_16_regmap_config.

Legacy functions and code will be removed once all consumers have
migrated to the new idio-16 library interface.

For IDIO-16 devices we have the following IRQ registers:

    Base Address +1 (Write): Clear Interrupt
    Base Address +2 (Read): Enable Interrupt
    Base Address +2 (Write): Disable Interrupt

An interrupt is asserted whenever a change-of-state is detected on any
of the inputs. Any write to 0x2 will disable interrupts, while any read
will enable interrupts. Interrupts are cleared by a write to 0x1.

For 104-IDIO-16 devices, there is no IRQ status register, so software
has to assume that if an interrupt is raised then it was for the
104-IDIO-16 device.

For PCI-IDIO-16 devices, there is an additional IRQ register:

    Base Address +6 (Read): Interrupt Status

Interrupt status can be read from 0x6 where bit 2 set indicates that an
IRQ has been generated.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/b45081958ab53dfa697f4a8b15f1bfba46718068.1680618405.git.william.gray@linaro.org/
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

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