leds: lm3532: select regmap I2C API

Regmap APIs should be selected, otherwise link can fail

ERROR: modpost: "__devm_regmap_init_i2c" [drivers/leds/leds-lm3532.ko] undefined!

Fixes: bc1b8492 ("leds: lm3532: Introduce the lm3532 LED driver")
Cc: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>