This patch enables support for power-on sequencing of SDIO peripherals through DT.

In general, it's quite common that wifi modules and other similar
peripherals have several signals in addition to the SDIO interface that
needs wiggling before the module will power on. It's common to have a
reference clock, one or several power rails and one or several lines
for reset/enable type functions.

The binding as written today introduces a number of reset gpios,
a regulator and a clock specifier. The code will handle up to 2 gpio
reset lines, but it's trivial to increase to more than that if needed
at some point.

Implementation-wise, the MMC core has been changed to handle this during
host power up, before the host interface is powered on. I have not yet
implemented the power-down side, I wanted people to have a chance for
reporting back w.r.t. issues (or comments on the bindings) first.

I have not tested the regulator portion, since the system and module
I'm working on doesn't need one (Samsung Chromebook with Marvell
8797-based wifi). Testing of those portions (and reporting back) would
be appreciated.

Signed-off-by: Olof Johansson <redacted>
---
 Documentation/devicetree/bindings/mmc/mmc.txt |   11 +++++++
 drivers/mmc/core/core.c                       |   42 +++++++++++++++++++++++++
 drivers/mmc/core/host.c                       |   30 +++++++++++++++++-
 include/linux/mmc/host.h                      |    5 +++
 4 files changed, 87 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/mmc/mmc.txt b/Documentation/devicetree/bindings/mmc/mmc.txt
index 458b57f..962e0ee 100644
--- a/Documentation/devicetree/bindings/mmc/mmc.txt
+++ b/Documentation/devicetree/bindings/mmc/mmc.txt

@@ -5,6 +5,8 @@ these definitions.
 Interpreted by the OF core:
 - reg: Registers location and length.
 - interrupts: Interrupts used by the MMC controller.
+- clocks: Clocks needed for the host controller, if any.
+- clock-names: Goes with clocks above.
 
 Card detection:
 If no property below is supplied, host native card detect is used.

@@ -30,6 +32,15 @@ Optional properties:
 - cap-sdio-irq: enable SDIO IRQ signalling on this interface
 - full-pwr-cycle: full power cycle of the card is supported
 
+Card power and reset control:
+The following properties can be specified for cases where the MMC
+peripheral needs additional reset, regulator and clock lines. It is for
+example common for WiFi/BT adapters to have these separate from the main
+MMC bus:
+  - card-reset-gpios: Specify GPIOs for card reset (reset active low)