platform/chrome: wilco_ec: Add h1_gpio status to debugfs

platform/chrome: wilco_ec: Add h1_gpio status to debugfs

As part of Chrome OS's FAFT (Fully Automated Firmware Testing)
tests, we need to ensure that the H1 chip is properly setting
some GPIO lines. The h1_gpio attribute exposes the state
of the lines:
- ENTRY_TO_FACT_MODE in BIT(0)
- SPI_CHROME_SEL in BIT(1)

There are two reasons that I am exposing this in debugfs,
and not as a GPIO:
1. This is only useful for testing, so end users shouldn't ever
care about this. In fact, if it passes the tests, then the value of
h1_gpio will always be 2, so it would be really uninteresting for users.
2. This GPIO is not connected to, controlled by, or really even related
to the AP. The GPIO runs between the EC and the H1 security chip.

Changes in v4:
- Use "0x02x\n" instead of "02x\n" for format string
- Use DEFINE_DEBUGFS_ATTRIBUTE()
- Add documentation
Changes in v3:
- Fix documentation to correspond with formatting change in v2.
Changes in v2:
- Zero out the unused fields in the request.
- Format result as "%02x\n" instead of as a decimal.

Signed-off-by: Nick Crews <ncrews@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>