pinctrl: Add driver for Zynq

This adds a pin-control driver for Zynq.

Changes since v2:
- driver-specific DT properties are passed to the core in two arrays,
  one for the actual DT parsing one for the debugfs representation.
  Issue a compiler warning when the number of entries is not the same
  for both arrays.

Changes since v1:
 - fix EMIO_SD1_CD pin name
 - add USB to pinmux options

changes since RFCv2:
 - let Zynq select PINCTRL_ZYNQ. Boot hangs when pinctrl information is
   present in DT but no driver available.
 - add #defines to get rid of magical constants
 - add commas at end of initializers
 - separate changes in mach-zynq in separate patch
 - add driver specific io-standard DT property
 - refactored pinconf set function to not require arguments for
   argument-less properties
 - squash other patches in
   - support for IO-standard property
   - support for low-power mode property
   - migration to pinconf_generic_dt_node_to_map_all()
 - use newly created infrastructure to add pass driver-specific DT
   params to pinconf-generic

changes since RFC:
 - use syscon/regmap to access registers in SLCR space
 - rebase to 3.18: rename enable -> set_mux
 - add kernel-doc
 - support pinconf
   - supported attributes
     - pin-bias: pull up, tristate, disable
     - slew-rate: 0 == slow, 1 == fast; generic pinconf does not display
       argument

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Tested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>