mmc: pwrseq: Document DT bindings for the simple MMC power sequence

To support SOCs which specifies specific MMC power sequences, document
some MMC DT bindings to be able to describe these hardwares.

Let's also document bindings for a simple MMC power sequence provider,
which purpose is to support a set of common properties between various
SOCs.

In this initial step, let's also document a top level description of
the MMC power sequence and describe the compatible string used for the
simple MMC power sequence provider.

The simple MMC power sequence provider will initially support a reset
GPIO. From several earlier posted patches, it's clear that such
hardware exists. Especially some WLAN chips which are attached to an
SDIO interface may use a GPIO reset.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>

Documentation/devicetree/bindings/mmc/mmc-pwrseq-simple.txt

@@ -0,0 +1,20 @@
+* The simple MMC power sequence provider
+
+The purpose of the simple MMC power sequence provider is to supports a set of
+common properties between various SOC designs. It thus enables us to use the
+same provider for several SOC designs.
+
+Required properties:
+- compatible : contains "mmc-pwrseq-simple".
+
+Optional properties:
+- reset-gpios : contains a GPIO specifier. The reset GPIO is asserted at
+	initialization and prior we start the power up procedure of the card. It
+	will be de-asserted right after the power has been provided to the card.
+
+Example:
+
+	sdhci0_pwrseq {
+		compatible = "mmc-pwrseq-simple";
+		reset-gpios = <&gpio1 12 0>;
+	}

Documentation/devicetree/bindings/mmc/mmc.txt

@@ -65,6 +65,18 @@ Optional SDIO properties:
 - enable-sdio-wakeup: Enables wake up of host system on SDIO IRQ assertion
 
 
+MMC power sequences:
+--------------------
+
+System on chip designs may specify a specific MMC power sequence. To
+successfully detect an (e)MMC/SD/SDIO card, that power sequence must be
+maintained while initializing the card.
+
+Optional property:
+- mmc-pwrseq: phandle to the MMC power sequence node. See "mmc-pwrseq-*"
+	for documentation of MMC power sequence bindings.
+
+
 Use of Function subnodes
 ------------------------
 
@@ -101,6 +113,7 @@ sdhci@ab000000 {
 	max-frequency = <50000000>;
 	keep-power-in-suspend;
 	enable-sdio-wakeup;
+	mmc-pwrseq = <&sdhci0_pwrseq>
 }
 
 Example with sdio function subnode:
@@ -114,6 +127,7 @@ mmc3: mmc@01c12000 {
 	vmmc-supply = <&reg_vmmc3>;
 	bus-width = <4>;
 	non-removable;
+	mmc-pwrseq = <&sdhci0_pwrseq>
 	status = "okay";
 
 	brcmf: bcrmf@1 {