host: x4xx: Implement GPIO API

This implements the GPIO API for X410 through get_gpio_attr and
set_gpio_attr. In ATR mode, which channel's ATR state is chosen by the
set_gpio_src call, setting e.g. DB0_RF0 for channel 0 or DB0_RF1 for
channel 1. In manual mode, all 24 bits (for both ports) are set in
a single register write.

Although the front panel of the device has two ports, labelled GPIO0 and
GPIO1, this API exposes them as though they were a single 24-bit GPIO
port.

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