HID: mcp2221: add GPIO functionality support

MCP2221 has 4 pins that can be used as GPIO or configured
for alternate functionality such as clock generation and
IRQ detection. This patch adds support for GPIO functionality.

To set direction of a pin or to toggle its state after it
has been configured as GPIO, driver sends command to mcp2221
and parses response received from mcp2221. Based on this
response either 0 or appropriate error code is returned to
GPIO framework.

To get the direction or current state of a pin, driver
sends command and read response from the device. Based on
the response received from device direction or value
is sent to the GPIO framework.

Command from driver to mcp2221 device are output report.
Response received from mcp2221 is input report.

Datasheet (page 45-48) contains details about how to decode
the response received from device:
http://ww1.microchip.com/downloads/en/DeviceDoc/20005565B.pdf

Signed-off-by: Rishi Gupta <gupt21@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>