hw/gpio: introduce pcf8574 driver

NXP PCF8574 and compatible ICs are simple I2C GPIO expanders.
PCF8574 incorporates quasi-bidirectional IO, and simple
communication protocol, when IO read is I2C by

hw/gpio: introduce pcf8574 driver

NXP PCF8574 and compatible ICs are simple I2C GPIO expanders.
PCF8574 incorporates quasi-bidirectional IO, and simple
communication protocol, when IO read is I2C byte read, and
IO write is I2C byte write. User can think of it as
open-drain port, when line high state is input and line low
state is output.

Signed-off-by: Dmitrii Sharikhin <d.sharikhin@yadro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <f1552d822276e878d84c01eba2cf2c7c9ebdde00.camel@yadro.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

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