regmap: Add kerneldoc for struct regmap_config

struct i2c_client;

struct spi_device;

/**

 * Configuration for the register map of a device.

 *

 * @reg_bits: Number of bits in a register address, mandatory.

 * @val_bits: Number of bits in a register value, mandatory.

 */

struct regmap_config {

	int reg_bits;

	int val_bits;