During board verification, there is a need to test the various supported
eMMC/SD speed modes.  However, since the framework chooses the best mode
supported by the card and the host controller's caps, this currently
necessitates changing the devicetree for every iteration.

Allow the various speed mode host capabilities to be modified via
debugfs in order to allow easier hardware verification.  The values to
be written are the raw MMC_CAP* values from include/linux/mmc/host.h.
This is rather low-level, and these defines are not guaranteed to be
stable, but it is perhaps good enough for the intended use case.

MMC_CAP_AGGRESSIVE_PM can also be set, in order to be able to
re-initialize the card without having to physically remove and re-insert
it.

 /sys/kernel/debug/mmc0# grep timing ios
 timing spec:	9 (mmc HS200)

 // Turn on MMC_CAP_AGGRESSIVE_PM and re-trigger runtime suspend
 /sys/kernel/debug/mmc0# echo $(($(cat caps) | (1 << 7))) > caps
 /sys/kernel/debug/mmc0# echo on > /sys/bus/mmc/devices/mmc0\:0001/power/control
 /sys/kernel/debug/mmc0# echo auto > /sys/bus/mmc/devices/mmc0\:0001/power/control

 // MMC_CAP2_HS200_1_8V_SDR
 /sys/kernel/debug/mmc0# echo $(($(cat caps2) & ~(1 << 5))) > caps2
 /sys/kernel/debug/mmc0# echo on > /sys/bus/mmc/devices/mmc0\:0001/power/control
 /sys/kernel/debug/mmc0# grep timing ios
 timing spec:	8 (mmc DDR52)
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20230929-mmc-caps-v2-2-11a4c2d94f15@axis.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>