pinctrl: sandbox: add sandbox pinctrl driver

This driver actually does nothing but test pinctrl uclass, and
demonstrate how things work.

To try this driver, uncomment /* #define DEBUG */ in the
drivers/pinctrl/pinctrl-sandbox.c, and debug messages will be
displayed.

  DRAM:  128 MiB
  sandbox pinmux: group = 1 (serial_a), function = 1 (serial)
  Using default environment

  In:    cros-ec-keyb
  Out:   lcd
  Err:   lcd
  Net:   Net Initialization Skipped
  eth0: eth@10002000, eth1: eth@80000000, eth5: eth@90000000
  => i2c dev 0
  Setting bus to 0
  sandbox pinmux: group = 0 (i2c), function = 0 (i2c)
  sandbox pinconf: group = 0 (i2c), param = 3, arg = 1

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>