drivers: i2c_dw: Remove hard-coded instance count assumption

Introduce a Kconfig variable that the SoC can set to indicate the
number of instances in Device Tree. This also fixes the accuracy of
the Elkhart Lake instance count where the code was previously assuming
up to 12 instances even though DT lists 15 nodes.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>