i2c: acpi: Add an i2c_acpi_client_count() helper function

i2c: acpi: Add an i2c_acpi_client_count() helper function

We have 3 files now which have the need to count the number of
I2cSerialBus resources in an ACPI-device's resource-list.

Currently all implement their own helper function for this,
add a generic helper function to replace the 3 implementations.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210803160044.158802-2-hdegoede@redhat.com
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Wolfram Sang <wsa@kernel.org>