gpio.4: document device hints common to all devices on gpiobus

"at" keyword is documented in device.hints(5) for all buses, but it does
hurt to add another reference to it.
"pins" keyword is specific to gpiobus.
At least these two hints should be configured for any gpiobus device on
a hints based system.

MFC after:	10 days

share/man/man4/gpio.4

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 5, 2013
+.Dd June 26, 2019
 .Dt GPIO 4
 .Os
 .Sh NAME
@@ -103,12 +103,36 @@ passed to the kernel, being either statically compiled in, or by a variety
 of ways where the boot loader (or Open Firmware enabled system) passes the
 DTS blob to the kernel at boot.
 .Pp
+On a
+.Xr device.hints 5
+based system these hints can be used to configure drivers for devices
+attached to
+.Nm
+pins:
+.Bl -tag -width ".Va hint.driver.unit.pins"
+.It Va hint.driver.unit.at
+The
+.Nm gpiobus
+where the device is attached.
+For example,
+.Qq gpiobus0 .
+.Ar driver
+and
+.Ar unit
+are the driver name and the unit number for the device driver.
+.It Va hint.driver.unit.pins
+This is a bitmask of the pins on the
+.Nm gpiobus
+that are connected to the device.
+The pins will be allocated to the specified driver instance.
+.El
+.Pp
 The following
 .Xr device.hints 5
 are only provided by the
 .Cd ar71xx_gpio
 driver:
-.Bl -tag -width ".Va hint.gpioiic.%d.atXXX"
+.Bl -tag -width ".Va hint.gpio.function_clear"
 .It Va hint.gpio.%d.pinmask
 This is a bitmask of pins on the GPIO board that we would like to expose
 for use to the host operating system.
@@ -133,6 +157,7 @@ of some device in a system.
 .Xr gpioiic 4 ,
 .Xr gpioled 4 ,
 .Xr iicbus 4 ,
+.Xr device.hints 5 ,
 .Xr gpioctl 8
 .Sh HISTORY
 The