The pfc in the R8A7790 (and probably others in the R-Car gen 2 family)
supports switching SDHI signals between 3.3V and 1.8V voltage, and the
SD driver should do that when selecting a higher-speed mode.

Add a flag for pins that support low voltage mode and SoC operations to
get and set it.  Implement the pinconf power-source parameter using these
operations.

Signed-off-by: Ben Hutchings <redacted>
---
 .../bindings/pinctrl/renesas,pfc-pinctrl.txt       |  4 ++-
 drivers/pinctrl/sh-pfc/pinctrl.c                   | 34 ++++++++++++++++++++--
 drivers/pinctrl/sh-pfc/sh_pfc.h                    |  6 ++++
 3 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/Documentation/devicetree/bindings/pinctrl/renesas,pfc-pinctrl.txt b/Documentation/devicetree/bindings/pinctrl/renesas,pfc-pinctrl.txt
index bfe72ec055e3..93651742f1c1 100644
--- a/Documentation/devicetree/bindings/pinctrl/renesas,pfc-pinctrl.txt
+++ b/Documentation/devicetree/bindings/pinctrl/renesas,pfc-pinctrl.txt

@@ -69,7 +69,9 @@ Pin Configuration Node Properties:

 The pin configuration parameters use the generic pinconf bindings defined in
 pinctrl-bindings.txt in this directory. The supported parameters are
-bias-disable, bias-pull-up and bias-pull-down.
+bias-disable, bias-pull-up, bias-pull-down and power-source. For pins that
+can operate with either 3.3V or 1.8V signals, the power-source value should
+be 0 for 3.3V or 1 for 1.8V.

@@ -121,6 +125,8 @@ struct sh_pfc_soc_operations {
        unsigned int (*get_bias)(struct sh_pfc *pfc, unsigned int pin);
        void (*set_bias)(struct sh_pfc *pfc, unsigned int pin,
                         unsigned int bias);
+       bool (*get_low_voltage)(struct sh_pfc *pfc, unsigned int pin);
+       void (*set_low_voltage)(struct sh_pfc *pfc, unsigned int pin, bool low);
 };