i2c: kempld: Support ACPI I2C device declaration

Adds an ACPI companion to the KEMPLD I2C driver so that it correctly
detects any I2C devices nested under the KEMPLD's ACPI node (SBRG.CPLD).

This allows I2C devices attached to the KEMPLD I2C adapter to be declared
and instantiated via ACPI.

Signed-off-by: Chris Pringle <chris.pringle@phabrix.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>