pinctrl/amd: save pin registers over suspend/resume

The touchpad in the Asus laptop models X505BA/BP and X542BA/BP is
unresponsive after suspend/resume. The following error appears during
resume:

  i2c_hid i2c-ELAN1300:00: failed to reset device.

The problem here is that i2c_hid does not notice the interrupt being
generated at this point, because the GPIO is no longer configured
for interrupts.

Fix this by saving pinctrl-amd pin registers during suspend and
restoring them at resume time.

Based on code from pinctrl-intel.

Cc: stable@vger.kernel.org
Signed-off-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>