pinctrl: baytrail: Add pinconf group + function for the pmu_clk

On the Lenovo Yoga Tablet 2 830 / 1050 / 1051 models the 32KHz PMU clk,
which can be muxed externally to SUS pin 5 and/or 6 is used as a clock
for the audio codec.

On the 830 and 1050 models, with ship with Android as factory OS the
pin-muxing for this is not setup by the BIOS.

Add a pinconf group + function for the pmu_clk on SUS pin 5 and 6 to
allow setting the pinmux up from within the x86-android-tablets
platform code.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220223133153.730337-2-hdegoede@redhat.com