When retrieveing CD (card detect) and WP (write protect)
GPIO handles from the device tree, make sure to assign
them active low by default unless the "cd-inverted" or
"wp-inverted" properties are set. These properties mean
that respective signal is active HIGH since the SDHCI
specification stipulates that this kind of signals
should be treated as active LOW.

If the twocell GPIO flag is also specified as active
low, well that's nice and we will silently ignore the
tautological specification.

If however the GPIO line is specified as active low
in the GPIO flasg cell and "cd-inverted" or "wp-inverted"
is also specified, the latter takes precedence and we
print a warning.

The current effect on the MMC slot-gpio core are as
follows:

For CD GPIOs: no effect. The current code in
mmc/core/host.c calls mmc_gpiod_request_cd() with
the "override_active_level" argument set to true,
which means that whatever the GPIO descriptor
thinks about active low/high will be ignored, the
core will use the MMC_CAP2_CD_ACTIVE_HIGH to keep
track of this and reads the raw value from the
GPIO descriptor, totally bypassing gpiolibs inversion
semantics. I plan to clean this up at a later point
passing the handling of inversion semantics over
to gpiolib, so this patch prepares the ground for
that.

Fow WP GPIOs: this is probably fixing a bug, because
the code in mmc/core/host.c calls mmc_gpiod_request_ro()
with the "override_active_level" argument set to false,
which means it will respect the inversion semantics of
the gpiolib and ignore the MMC_CAP2_RO_ACTIVE_HIGH
flag for everyone using this through device tree.
However the code in host.c confusingly goes to great
lengths setting up the MMC_CAP2_RO_ACTIVE_HIGH flag
from the GPIO descriptor and by reading the "wp-inverted"
property of the node. As far as I can tell this is all
in vain and the inversion is broken: device trees that
use "wp-inverted" do not work as intended, instead the
only way to actually get inversion on a line is by
setting the second cell flag to GPIO_ACTIVE_HIGH (which
will be the default) or GPIO_ACTIVE_LOW if they want
the proper MMC semantics. Presumably all device trees do
this right but we need to parse and handle this properly.

Cc: linux-mmc@vger.kernel.org
Cc: linux-gpio@vger.kernel.org
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>