mmc: core: make pwrseq_emmc (partially) support sleepy GPIO controllers

pwrseq_emmc.c implements a HW reset procedure for eMMC chip by driving a
GPIO line.

It registers the .reset() cb on mmc_pwrseq_ops and it registers a system
restart notification handler; both of them perform reset by unconditionally
calling gpiod_set_value().

If the eMMC reset line is tied to a GPIO controller whose driver can sleep
(i.e. I2C GPIO controller), then the kernel would spit warnings when trying
to reset the eMMC chip by means of .reset() mmc_pwrseq_ops cb (that is
exactly what I'm seeing during boot).

Furthermore, on system reset we would gets to the system restart
notification handler with disabled interrupts - local_irq_disable() is
called in machine_restart() at least on ARM/ARM64 - and we would be in
trouble when the GPIO driver tries to sleep (which indeed doesn't happen
here, likely because in my case the machine specific code doesn't call
do_kernel_restart(), I guess..).

This patch fixes the .reset() cb to make use of gpiod_set_value_cansleep(),
so that the eMMC gets reset on boot without complaints, while, since there
isn't that much we can do, we avoid register the restart handler if the
GPIO controller has a sleepy driver (and we spit a dev_notice() message to
let people know)..

This had been tested on a downstream 4.9 kernel with backported
commit 83f37ee7ba33 ("mmc: pwrseq: Add reset callback to the struct
mmc_pwrseq_ops") and commit ae60fb031cf2 ("mmc: core: Don't do eMMC HW
reset when resuming the eMMC card"), because I couldn't boot my board
otherwise. Maybe worth to RFT.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>