pinctrl: Add pinmux property support to pinctrl-generic

pinctrl: Add pinmux property support to pinctrl-generic

The pinmux property allows for smaller and more compact device trees,
especially when there are many pins which need to be assigned individually.
Instead of specifying an array of strings to be parsed as pins and a
function property, the pinmux property contains an array of integers
representing pinmux groups. A pinmux group consists of the pin identifier
and mux settings represented as a single integer or an array of integers.
Each individual pin controller driver specifies the exact format of a
pinmux group. As specified in the Linux documentation, a pinmux group may
be multiple integers long. However, no existing drivers use multi-integer
pinmux groups, so I have chosen to omit this feature. This makes the
implementation easier, since there is no need to allocate a buffer to do
endian conversions.

Support for the pinmux property is done differently than in Linux.  As far
as I can tell, inversion of control is used when implementing support for
the pins and groups properties to avoid allocating. This results in some
duplication of effort; every property in a config node is parsed once for
each pin in that node. This is not such an overhead with pins and groups
properties, since having multiple pins in one config node does not occur
especially often. However, the semantics of the pinmux property make such a
configuration much more appealing. A future patch could parse all config
properties at once and store them in an array. This would make it easier to
create drivers which do not function solely as callbacks from
pinctrl-generic.

This commit increases the size of the sandbox build by approximately 48
bytes.  However, it also decreases the size of the K210 device tree by 2
KiB from the previous version of this series.

The documentation has been updated from the last Linux commit before it was
split off into yaml files.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>