i2c: gpio: Enable working over slow can_sleep GPIOs

"Slow" GPIOs (usually those connected over an SPI or an I2C bus) are,
well, slow in their operation. It is generally a good idea to avoid
using them for time-critical operation, but sometimes the hardware just
sucks, and the software has to cope. In addition to that, the I2C bus
itself does not actually define any strict timing limits; the bus is
free to go all the way down to DC. The timeouts (and therefore the
slowest acceptable frequency) are present only in SMBus.

The `can_sleep` is IMHO a wrong concept to use here. My SPI-to-quad-UART
chip (MAX14830) is connected via a 26MHz SPI bus, and it happily drives
SCL at 200kHz (5µs pulses) during my benchmarks. That's faster than the
maximal allowed speed of the traditional I2C.

The previous version of this code did not really block operation over
slow GPIO pins, anyway. Instead, it just resorted to printing a warning
with a backtrace each time a GPIO pin was accessed, thereby slowing
things down even more.

Finally, it's not just me. A similar patch was originally submitted in
2015 [1].

[1] https://patchwork.ozlabs.org/patch/450956/

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

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